fix(studio): chain config dir + spawn path + doc URLs

Track the platform rename: the hub spawn path is now ~/.chain/bin/chain
(was ~/.fai/bin/fai.exe on Windows — both dir and binary were stale, so
Studio could not launch the hub after the config-dir rename), the
~/.fai/* help strings become ~/.chain/*, FAI_REGISTRY_TOKEN ->
CHAIN_REGISTRY_TOKEN, and the two in-app doc URLs point at the public
fai/chain repo (fai/platform was renamed to the private fai/chain-private).
The .fai module bundle extension is left unchanged (format phase).
flutter analyze: no issues.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_en.arb

@@ -40,7 +40,7 @@
   "errOutputTooLarge": "Step output exceeded the size limit.",
   "errOutputTooLargeHint": "Either trim the output (truncate text, downsample bytes) or raise `max_output_size_mb` in the operator config.",
   "errServiceUnavailableForStep": "Host service not configured.",
-  "errServiceUnavailableForStepHint": "The module requires a service (e.g. ollama, playwright) the operator hasn't declared. Add it under `services:` in ~/.fai/config.yaml.",
+  "errServiceUnavailableForStepHint": "The module requires a service (e.g. ollama, playwright) the operator hasn't declared. Add it under `services:` in ~/.chain/config.yaml.",
   "errMissingValue": "Required value is missing.",
   "errMissingValueHint": "The step expected an input that wasn't wired up — either supply it at run time or fix the upstream `$ref`.",
   "errMcpUnreachable": "MCP endpoint unreachable.",
@@ -151,7 +151,7 @@
   "settingsHubEndpointHint": "Where should Studio connect? Default is the local hub at 127.0.0.1:50051.",
   "settingsTlsSubtitle": "https/grpc-secure",
   "settingsPortError": "port must be 1–65535",
-  "channelsBlurb": "Switch hub channel (writes ~/.fai/current-channel and restarts the daemon). Connect just changes Studio's wire.",
+  "channelsBlurb": "Switch hub channel (writes ~/.chain/current-channel and restarts the daemon). Connect just changes Studio's wire.",
   "channelsActionsTooltip": "Channel actions",
   "systemAiHeader": "SYSTEM AI",
   "systemAiEnabled": "enabled",
@@ -436,7 +436,7 @@
   "storeOpenRepoButton": "Open repository in browser",
   "storeDocsErrorNotFound": "This module is not in the hub's store index.",
   "storeDocsErrorNoDocs": "No repository URL is recorded for this module.",
-  "storeDocsErrorAuth": "The registry requires authentication to fetch this README. Set FAI_REGISTRY_TOKEN in the daemon's environment and restart the hub.",
+  "storeDocsErrorAuth": "The registry requires authentication to fetch this README. Set CHAIN_REGISTRY_TOKEN in the daemon's environment and restart the hub.",
   "storeDocsErrorNetwork": "Network error while fetching the README. {detail}",
   "@storeDocsErrorNetwork": {
     "placeholders": {
@@ -610,7 +610,7 @@
   "auditClearLogButton": "Clear log",
   "modulesTitle": "Modules",
   "modulesNoneTitle": "No modules yet",
-  "modulesNoneHint": "Run `chain install <capability-name>` or check ~/.fai/modules/.",
+  "modulesNoneHint": "Run `chain install <capability-name>` or check ~/.chain/modules/.",
   "modulesReloadTooltip": "Reload",
   "modulesRecentActivity": "RECENT ACTIVITY",
   "modulesRecentActivityHint": "last {n} install/uninstall events from the audit log",
@@ -761,7 +761,7 @@
     }
   },
   "registryCredentialsHeader": "REGISTRY CREDENTIALS",
-  "registryCredentialsBlurb": "Token used to download .fai modules from a registry behind a signin wall (Forgejo, GitHub-private). Stored at ~/.fai/registry-token, mode 0600. The FAI_REGISTRY_TOKEN env var still wins when set.",
+  "registryCredentialsBlurb": "Token used to download .fai modules from a registry behind a signin wall (Forgejo, GitHub-private). Stored at ~/.chain/registry-token, mode 0600. The CHAIN_REGISTRY_TOKEN env var still wins when set.",
   "registryTokenStatusConfigured": "Configured ({chars} chars)",
   "@registryTokenStatusConfigured": {
     "placeholders": {
@@ -775,7 +775,7 @@
   "registryTokenFieldHint": "Paste your Forgejo or GitHub personal access token",
   "registryTokenSaveButton": "Save",
   "registryTokenClearButton": "Clear",
-  "registryTokenStorageHint": "Saved locally to ~/.fai/registry-token. Never sent to a remote service.",
+  "registryTokenStorageHint": "Saved locally to ~/.chain/registry-token. Never sent to a remote service.",
   "registryTokenSavedToast": "Registry token saved.",
   "registryTokenClearedToast": "Registry token cleared.",
   "registryTokenSaveFailedToast": "Could not save: {error}",
@@ -787,7 +787,7 @@
     }
   },
   "hubAuthTokenHeader": "HUB AUTHENTICATION",
-  "hubAuthTokenBlurb": "Bearer token used to authenticate Studio against a hub that has auth.tokens: configured (RBAC Level 2). Stored at ~/.fai/hub-auth-token, mode 0600. Studio sends it as Authorization: Bearer on every gRPC call.",
+  "hubAuthTokenBlurb": "Bearer token used to authenticate Studio against a hub that has auth.tokens: configured (RBAC Level 2). Stored at ~/.chain/hub-auth-token, mode 0600. Studio sends it as Authorization: Bearer on every gRPC call.",
   "hubAuthTokenStatusConfigured": "Configured ({chars} chars)",
   "@hubAuthTokenStatusConfigured": {
     "placeholders": {
@@ -801,7 +801,7 @@
   "hubAuthTokenFieldHint": "Paste a token from the hub operator's auth.tokens entry",
   "hubAuthTokenSaveButton": "Save",
   "hubAuthTokenClearButton": "Clear",
-  "hubAuthTokenStorageHint": "Saved locally to ~/.fai/hub-auth-token. Studio reconnects after save so the new token takes effect immediately.",
+  "hubAuthTokenStorageHint": "Saved locally to ~/.chain/hub-auth-token. Studio reconnects after save so the new token takes effect immediately.",
   "hubAuthTokenSavedToast": "Hub auth token saved.",
   "hubAuthTokenClearedToast": "Hub auth token cleared.",
   "hubAuthTokenSaveFailedToast": "Could not save: {error}",
@@ -833,15 +833,15 @@
   "defaultScopeLoadFailed": "Could not load default scope.",
   "defaultScopeNonEmptyError": "Default scope cannot be empty — keep at least one segment.",
   "maintenanceHeader": "HUB MAINTENANCE",
-  "maintenanceResetBlurb": "Wipe operator state and start over with a clean hub. Stops every running daemon, atomically backs ~/.fai/ up, then recreates it with the binary, channel state, MCP / n8n / system-AI config, and registry token preserved. Restore by moving the backup directory back.",
+  "maintenanceResetBlurb": "Wipe operator state and start over with a clean hub. Stops every running daemon, atomically backs ~/.chain/ up, then recreates it with the binary, channel state, MCP / n8n / system-AI config, and registry token preserved. Restore by moving the backup directory back.",
   "maintenanceKeepModulesTitle": "Keep installed modules",
-  "maintenanceKeepModulesSubtitle": "Default behaviour wipes ~/.fai/modules/ so a fresh install can verify the module-fetch path end-to-end.",
+  "maintenanceKeepModulesSubtitle": "Default behaviour wipes ~/.chain/modules/ so a fresh install can verify the module-fetch path end-to-end.",
   "maintenanceKeepDataTitle": "Keep audit log + saved flows",
-  "maintenanceKeepDataSubtitle": "Default behaviour wipes ~/.fai/data/ so the audit chain restarts from a fresh genesis and the bundled sample flows re-import on first daemon start.",
+  "maintenanceKeepDataSubtitle": "Default behaviour wipes ~/.chain/data/ so the audit chain restarts from a fresh genesis and the bundled sample flows re-import on first daemon start.",
   "maintenanceResetButton": "Reset hub state",
   "maintenanceResetInProgress": "Resetting…",
   "maintenanceResetConfirmTitle": "Reset hub state?",
-  "maintenanceResetConfirmBody": "This stops every daemon, moves ~/.fai/ to a timestamped backup, and starts fresh. The backup is fully restorable. Continue?",
+  "maintenanceResetConfirmBody": "This stops every daemon, moves ~/.chain/ to a timestamped backup, and starts fresh. The backup is fully restorable. Continue?",
   "maintenanceResetConfirmButton": "Reset",
   "welcomeChecklistAllSetTitle": "You're set up.",
   "welcomeChecklistAllSetBody": "Three threads to pull on next:",
@@ -951,15 +951,15 @@
   "errNotFoundHint": "The item you asked for isn't here — it may have been removed or never existed.",
   "errAlreadyExists": "Already exists.",
   "errPermissionDenied": "Permission denied.",
-  "errPermissionDeniedHint": "The hub's operator policy blocks this action. Check `~/.fai/config.yaml`.",
+  "errPermissionDeniedHint": "The hub's operator policy blocks this action. Check `~/.chain/config.yaml`.",
   "errFailedPrecondition": "Setup issue blocked this.",
   "errFailedPreconditionHint": "The hub couldn't proceed because something it needs isn't ready. The detail below explains what.",
   "errInternal": "Unexpected internal error.",
-  "errInternalHint": "Check the hub logs at `~/.fai/logs/`. If this repeats, please report it.",
+  "errInternalHint": "Check the hub logs at `~/.chain/logs/`. If this repeats, please report it.",
   "errUnavailable": "Hub not reachable.",
   "errUnavailableHint": "Make sure `chain serve` (or `chain daemon start`) is running and the daemon endpoint matches Studio's settings.",
   "errUnauthenticated": "Authentication required.",
-  "errUnauthenticatedHint": "Set `FAI_REGISTRY_TOKEN` or sign in through Settings before retrying.",
+  "errUnauthenticatedHint": "Set `CHAIN_REGISTRY_TOKEN` or sign in through Settings before retrying.",
   "errCopyDetail": "Copy detail",
   "errDetailCopied": "Detail copied to clipboard.",
   "flowsOutputUnknown": "Output payload variant not recognised.",
@@ -1114,7 +1114,7 @@
   },
   "doctorApprovalsAttentionPill": "attention",
   "doctorServicesEmpty": "No host services declared",
-  "doctorServicesEmptyHint": "add to ~/.fai/config.yaml under services:",
+  "doctorServicesEmptyHint": "add to ~/.chain/config.yaml under services:",
   "doctorPillLoaded": "loaded",
   "doctorPillEmpty": "empty",
   "doctorRecheckTooltip": "Re-check",
@@ -1480,7 +1480,7 @@
   "settingsIntegrationsPanelTitle": "Integrations",
   "settingsIntegrationsPanelBody": "External tool servers the hub federates capabilities from. MCP servers expose tool-shaped endpoints; n8n endpoints expose hosted workflows.",
   "settingsSecurityPanelTitle": "Security & credentials",
-  "settingsSecurityPanelBody": "Tokens the hub holds on the operator's behalf. Stored in ~/.fai/ with mode 0600; never sent in telemetry.",
+  "settingsSecurityPanelBody": "Tokens the hub holds on the operator's behalf. Stored in ~/.chain/ with mode 0600; never sent in telemetry.",
   "settingsMaintenancePanelTitle": "Maintenance",
   "settingsMaintenancePanelBody": "Destructive operations gated behind explicit confirm dialogs. Use when re-pilot-testing or debugging.",
   "tooltipMoveUp": "Move up",