feat(studio): trust pass — friendly errors + store clarity + MCP i18n

Second half of the May-2026 trust pass. Drops the wall of
gRPC trailers from every error surface and makes the Store
honest about what is and isn't installable.

Friendly errors:

- New `friendlyError(Object, AppLocalizations)` mapper turns
  GrpcError + arbitrary throwables into a localised headline,
  optional recovery hint, and a verbatim detail string kept
  behind a "Show details" expander. Duck-typed on `.code` /
  `.message` so Studio doesn't have to depend on package:grpc
  directly.
- `FaiErrorBox` gains an `error:` constructor that runs the
  mapper. Every call site that used to render
  `snap.error.toString()` (flows, welcome, store) switches to
  it.
- 9 .arb entries per locale cover the gRPC codes we actually
  emit (INVALID_ARGUMENT, NOT_FOUND, ALREADY_EXISTS,
  PERMISSION_DENIED, FAILED_PRECONDITION, INTERNAL,
  UNAVAILABLE, UNAUTHENTICATED) plus copy/details affordances.
- `test/friendly_error_test.dart` — 6 unit tests for the
  mapper. Covers the mapping table, locale-switching, and the
  non-gRPC fallback so future regressions show up in CI.

Capability discovery:

- New `HubService.allCapabilities()` reads the kind-aware
  capability list (wasm + builtin + federated) and returns a
  Dart-side `CapabilityInfo` value type. The flow page's
  missing-dependency check uses it so `system.approval` and
  federated MCP/n8n tools count as "available" — fixes the
  Run button staying disabled forever.
- `HubService.listModules()` filters to kind=wasm so the
  Modules page doesn't sprout synthetic "system" entries that
  the operator can't uninstall.

Store clarity:

- New "Installable only" filter, on by default. Roughly 2/3
  of seed entries currently carry `status: planned`; the
  default view stops being noise.
- Featured-strip cards for planned modules now show a
  "Coming soon" pill instead of an empty action area.
- Main-grid cards for non-installable modules dim to 60%
  opacity so the eye lands on actionable cards first.
- Detail-sheet "Nicht installierbar" tooltip → inline hint
  box. The reason is visible without hovering.

MCP localisation:

- `_kMcpSuggestions` no longer holds 11 hardcoded English
  description strings. The `description` field is replaced
  with a `resolveDescription(AppLocalizations)` lookup that
  switches on the suggestion `name` to read the matching
  `mcpSuggestion*Desc` .arb key. EN + DE shipped.
- New `FaiEnBadge` widget renders a small `[EN]` pill when
  the active locale isn't English. Used next to MCP /
  federated store entries' tagline + description because
  the server supplies them in English and we can't translate
  on the fly yet — the badge is the honest signal until the
  planned `studio.translate` plugin lands.

Plus housekeeping: removed the unused `_keepImport` lint
escape in the test and the dangling library doc-comment in
`format.dart`.

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

lib/l10n/app_de.arb

@@ -75,6 +75,25 @@
   "buttonReadDocs": "Doku öffnen",
   "buttonCopy": "In Zwischenablage kopieren",
   "buttonCopied": "Kopiert",
+  "buttonShowDetails": "Details anzeigen",
+  "buttonHideDetails": "Details ausblenden",
+  "storeInstallableOnly": "Nur installierbare",
+  "storePillComingSoon": "Bald verfügbar",
+  "storeNotInstallableInline": "Dieses Modul ist auf der Roadmap (Status: {status}). Es gibt noch kein installierbares Bundle — Repository verfolgen für Updates.",
+  "@storeNotInstallableInline": { "placeholders": { "status": { "type": "String" } } },
+
+  "mcpSuggestionDeepwikiDesc": "Öffentliches HTTPS — KI-gestützte Doku-Suche in GitHub-Repos. Kein Node, kein API-Key.",
+  "mcpSuggestionSemgrepDesc": "Öffentliches HTTPS — Security-Scanning von Code-Schwachstellen. Kein Node, kein API-Key.",
+  "mcpSuggestionFilesystemDesc": "Anthropic — Dateien in /tmp lesen / schreiben. Pfad vor dem Speichern anpassen.",
+  "mcpSuggestionFetchDesc": "Anthropic — beliebige HTTP(S)-URLs als Markdown abrufen.",
+  "mcpSuggestionGithubDesc": "Anthropic — Issues, PRs, Repo-Dateien. Braucht einen GitHub-PAT in der Env-Variable.",
+  "mcpSuggestionPuppeteerDesc": "Anthropic — Headless-Browser-Automation für Screenshots / Scraping.",
+  "mcpSuggestionPostgresDesc": "Anthropic — Read-only-SQL-Queries gegen eine Postgres-Datenbank. URL anpassen.",
+  "mcpSuggestionSqliteDesc": "Anthropic — Abfragen gegen eine lokale SQLite-Datei. Pfad anpassen.",
+  "mcpSuggestionBraveSearchDesc": "Anthropic — Websuche via Brave. API-Key erforderlich.",
+  "mcpSuggestionMemoryDesc": "Anthropic — persistenter Knowledge-Graph als Agenten-Gedächtnis.",
+  "mcpSuggestionTimeDesc": "Anthropic — aktuelle Uhrzeit + Zeitzonen-Konvertierung.",
+  "mcpServerEnLanguageBadge": "Server-Text in Englisch — Studio zeigt ihn wie geliefert.",
 
   "settingsTitle": "Hub-Endpunkt",
   "settingsHost": "Host",
@@ -512,6 +531,25 @@
   "@flowsErrorTitle": { "placeholders": { "name": { "type": "String" } } },
   "flowsRunning": "Flow läuft…",
   "flowsNoOutputs": "Flow abgeschlossen ohne deklarierte Outputs.",
+
+  "errGeneric": "Da ist etwas schiefgegangen.",
+  "errInvalidArgument": "Ungültige Eingabe.",
+  "errInvalidArgumentHint": "Werte im Formular prüfen und erneut versuchen.",
+  "errNotFound": "Nicht gefunden.",
+  "errNotFoundHint": "Das Angefragte ist nicht hier — entweder entfernt oder nie existiert.",
+  "errAlreadyExists": "Existiert bereits.",
+  "errPermissionDenied": "Berechtigung verweigert.",
+  "errPermissionDeniedHint": "Die Operator-Policy des Hubs blockiert diese Aktion. `~/.fai/config.yaml` prüfen.",
+  "errFailedPrecondition": "Setup-Problem hat das blockiert.",
+  "errFailedPreconditionHint": "Der Hub konnte nicht weiter, weil etwas Vorausgesetztes fehlt. Details unten erklären was.",
+  "errInternal": "Unerwarteter interner Fehler.",
+  "errInternalHint": "Hub-Logs unter `~/.fai/logs/` prüfen. Wenn das öfter passiert, bitte melden.",
+  "errUnavailable": "Hub nicht erreichbar.",
+  "errUnavailableHint": "Sicherstellen, dass `fai serve` (oder `fai daemon start`) läuft und der Daemon-Endpunkt zu Studios Einstellungen passt.",
+  "errUnauthenticated": "Authentifizierung erforderlich.",
+  "errUnauthenticatedHint": "`FAI_REGISTRY_TOKEN` setzen oder in den Einstellungen anmelden, dann erneut versuchen.",
+  "errCopyDetail": "Detail kopieren",
+  "errDetailCopied": "Detail in die Zwischenablage kopiert.",
   "flowsOutputUnknown": "Unbekannter Output-Payload-Typ.",
   "flowsOutputBytesUnknownMime": "Binärdaten",
   "flowsOutputSaveAs": "Speichern unter…",