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/data/error_presentation.dart

@@ -6,7 +6,7 @@
 //   • a copyable, selectable rendition of the message
 //   • a friendly headline (gRPC codes mapped via
 //     friendlyError) instead of a wall-of-stack-trace
-//   • an automatic append to ~/.fai/logs/studio-errors.log so
+//   • an automatic append to ~/.chain/logs/studio-errors.log so
 //     the trail survives the dismiss action
 //
 // Call sites used to scatter `ScaffoldMessenger.of(ctx).showSnackBar(

lib/data/fai_log.dart

@@ -4,7 +4,7 @@
 // went wrong without the operator having to reproduce the
 // failure.
 //
-// Path: `~/.fai/logs/studio-errors.log` (cross-platform via
+// Path: `~/.chain/logs/studio-errors.log` (cross-platform via
 // HOME / USERPROFILE). One line per event, JSON-shaped so
 // `chain doctor` can parse it; humans can still read it because
 // the JSON is single-line and short.

lib/data/hub.dart

@@ -55,7 +55,7 @@ class HubService {
   }
 
   /// Reconnect to a new endpoint and persist for next launch.
-  /// [authToken] is read from `~/.fai/hub-auth-token` by
+  /// [authToken] is read from `~/.chain/hub-auth-token` by
   /// default — pass `null` to drop a previously-loaded token,
   /// or omit the parameter to keep the current value.
   Future<void> reconnect(
@@ -241,7 +241,7 @@ class HubService {
   }
 
   /// Persist a new System-AI configuration. The hub writes back
-  /// to `~/.fai/config.yaml` and hot-reloads the in-memory copy
+  /// to `~/.chain/config.yaml` and hot-reloads the in-memory copy
   /// in one round-trip; the caller gets the resulting status
   /// for an immediate UI refresh.
   Future<SystemAiStatus> updateSystemAi({
@@ -449,7 +449,7 @@ class HubService {
 
   /// Read the installed module's `MODULE.md` (or
   /// `MODULE.<locale>.md`) from disk under
-  /// `~/.fai/modules/<module>/`. Air-gap friendly, no network.
+  /// `~/.chain/modules/<module>/`. Air-gap friendly, no network.
   ///
   /// `state` is one of:
   ///   - `'found'`     — `text` holds the markdown

lib/data/hub_auth_token.dart

@@ -3,7 +3,7 @@ import 'package:path/path.dart' as p;
 
 /// Operator-managed gRPC bearer token used to authenticate
 /// Studio against a hub that has `auth.tokens:` configured
-/// (RBAC Level 2). Stored at `~/.fai/hub-auth-token` (mode
+/// (RBAC Level 2). Stored at `~/.chain/hub-auth-token` (mode
 /// 0600 on Unix). Sister of [RegistryToken] — same on-disk
 /// hygiene, different secret.
 ///
@@ -55,7 +55,7 @@ class HubAuthToken {
     return content.isEmpty ? null : content;
   }
 
-  /// Persist [token] to disk, creating `~/.fai/` if needed.
+  /// Persist [token] to disk, creating `~/.chain/` if needed.
   ///
   /// Atomic write: writes to `<path>.tmp`, chmods 0600 on
   /// Unix BEFORE moving into place, then renames. This
@@ -63,7 +63,7 @@ class HubAuthToken {
   /// `writeAsString` + later `chmod 600` sequence left the
   /// file world-readable for the duration of the chmod call.
   ///
-  /// Also sets `~/.fai/` itself to 0700 on Unix on first
+  /// Also sets `~/.chain/` itself to 0700 on Unix on first
   /// creation so other users on a shared host can't even
   /// enumerate the directory (the 0600 token-file mode is
   /// fine in isolation; combining it with a 0755 parent

lib/data/registry_token.dart

@@ -2,8 +2,8 @@ import 'dart:io';
 import 'package:path/path.dart' as p;
 
 /// Operator-managed registry auth token, kept at
-/// `~/.fai/registry-token` (mode 0600 on Unix). The hub reads
-/// this file at install time when `FAI_REGISTRY_TOKEN` is unset
+/// `~/.chain/registry-token` (mode 0600 on Unix). The hub reads
+/// this file at install time when `CHAIN_REGISTRY_TOKEN` is unset
 /// — see `download_to_temp` in `crates/fai_hub/src/lib.rs`.
 ///
 /// Studio writes the file directly so a fresh install never
@@ -40,7 +40,7 @@ class RegistryToken {
     return trimmed.isEmpty ? null : trimmed.length;
   }
 
-  /// Persist [token] to disk, creating `~/.fai/` if needed.
+  /// Persist [token] to disk, creating `~/.chain/` if needed.
   /// On Unix the file is chmod-ed to 0600 (owner read/write
   /// only). On Windows the default user-ACL is left alone —
   /// best-effort, no PowerShell handshake.

lib/data/system_actions.dart

@@ -11,7 +11,7 @@
 //   call for processes.
 //
 // The `fai` binary is resolved from PATH first; falls back to
-// `~/.fai/bin/chain` (Unix) or `%USERPROFILE%\.fai\bin\chain.exe`
+// `~/.chain/bin/chain` (Unix) or `%USERPROFILE%\.chain\bin\chain.exe`
 // (Windows). The PowerShell installer puts both in PATH but
 // some operators don't restart their shell after install — the
 // fallback covers that case.
@@ -30,7 +30,7 @@ const String kFaiBinaryNotFound = 'fai-binary-not-found';
 /// in the OS browser as the fallback affordance when no `fai`
 /// binary can be located on the machine.
 const String kFaiInstallDocsUrl =
-    'https://git.flemming.ai/fai/platform#installation';
+    'https://git.flemming.ai/fai/chain#installation';
 
 class SystemActions {
   SystemActions._();
@@ -151,7 +151,7 @@ class SystemActions {
     return _runFai(['update', 'apply', '--channel', channel]);
   }
 
-  /// Switch the active channel pointer at `~/.fai/current-channel`.
+  /// Switch the active channel pointer at `~/.chain/current-channel`.
   /// The CLI also restarts the daemon for the new channel, so the
   /// caller does not need a follow-up restart.
   static Future<({bool ok, String stdout, String stderr})> faiChannelSwitch(
@@ -179,7 +179,7 @@ class SystemActions {
 
   /// Run `chain reset --yes`. Wipes operator state with the same
   /// safety net as the CLI: stops every daemon (including off-
-  /// channel orphans), atomically backs `~/.fai/` up, recreates
+  /// channel orphans), atomically backs `~/.chain/` up, recreates
   /// it with `bin/`, `channels/`, `config.yaml`, `current-channel`,
   /// and `registry-token` preserved. The daemon restarts on the
   /// active channel before the call returns.
@@ -248,8 +248,8 @@ class SystemActions {
     final home = Platform.environment[isWindows ? 'USERPROFILE' : 'HOME'];
     if (home == null || home.isEmpty) return null;
     final fallback = isWindows
-        ? '$home\\.fai\\bin\\fai.exe'
-        : '$home/.fai/bin/chain';
+        ? '$home\\.chain\\bin\\chain.exe'
+        : '$home/.chain/bin/chain';
     return File(fallback).existsSync() ? fallback : null;
   }
 

lib/data/today_story_loader.dart

@@ -1,6 +1,6 @@
 // Today-Hero story loader.
 //
-// Reads ~/.fai/today/active.yaml when present, validates the
+// Reads ~/.chain/today/active.yaml when present, validates the
 // schema, and hands the parsed bilingual story back to the
 // store page. On any failure (file missing, schema mismatch,
 // banned-words hit, parse error) the caller falls back to the
@@ -73,7 +73,7 @@ class TodayStoryData {
 /// store page calls `loadOrFallback` once per fetch and feeds
 /// the result into `_StoreTodayHero`.
 class TodayStoryLoader {
-  /// Resolves to `~/.fai/today/active.yaml` on every supported
+  /// Resolves to `~/.chain/today/active.yaml` on every supported
   /// host. The pipeline writes here from [tools/today/accept.sh].
   static String activePath() {
     final home =