feat(studio): app-wide i18n via flutter_localizations (v0.24.0)

Replaces the Store-only DE/EN toggle with an app-wide one
parked in the sidebar footer next to the theme button.
Pressing it flips every translated string at once: nav
labels, page titles, common buttons, the bilingual
store-index content.

Implementation:
- Adds `flutter_localizations` + `intl` to pubspec, plus
  `flutter.generate: true` so `flutter gen-l10n` runs in the
  build pipeline.
- ARB sources at `lib/l10n/app_en.arb` and `app_de.arb`. The
  EN file is the template; DE carries the German strings.
  Initial coverage: navigation, common buttons, page titles,
  channels / store / audit / modules / approvals headers,
  hub-unreachable copy, MCP + n8n panel headers + hints.
  Rest of the UI strings are still English-literal — those
  fall in incrementally as we touch each surface.
- Generated `AppLocalizations` lives at
  `lib/l10n/app_localizations*.dart` (regenerated via
  `flutter gen-l10n` on every ARB edit).
- `StudioAppState` gains `localeNotifier` alongside
  `modeNotifier`; persisted via SharedPreferences key
  `locale.code`.
- Sidebar `_LanguageToggle` reads/writes through the
  notifier. The Store's per-page locale state is gone:
  `_locale` now reads `Localizations.localeOf(context)
  .languageCode`, so the bilingual store-index content
  follows the global setting without a second toggle.
- `_NavPage.label` becomes `_NavPage.id` + `labelOf(context)`;
  Cmd+K palette and Sidebar both read the localized label.

Out of scope this iteration: localizing the remaining
~80% of UI strings (Settings dialog labels, Store search
hint, error messages). Those land incrementally — the i18n
infrastructure now means each is a one-line ARB edit + one
call-site swap.

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

lib/l10n/app_de.arb

@@ -0,0 +1,93 @@
+{
+  "@@locale": "de",
+
+  "appTitle": "F∆I Studio",
+
+  "navDoctor": "Diagnose",
+  "navModules": "Module",
+  "navStore": "Store",
+  "navFlows": "Flows",
+  "navAudit": "Protokoll",
+  "navApprovals": "Freigaben",
+
+  "connectionConnected": "verbunden",
+  "connectionUnreachable": "nicht erreichbar",
+  "connectionConnecting": "verbinde…",
+  "connectionStartHub": "Hub starten",
+
+  "buttonCancel": "Abbrechen",
+  "buttonSave": "Speichern",
+  "buttonClose": "Schließen",
+  "buttonRetry": "Erneut versuchen",
+  "buttonRefresh": "Aktualisieren",
+  "buttonInstall": "Installieren",
+  "buttonUninstall": "Deinstallieren",
+  "buttonDetails": "Details",
+  "buttonAdd": "Hinzufügen",
+  "buttonRemove": "Entfernen",
+  "buttonOpen": "Öffnen",
+  "buttonExplain": "Erklären",
+  "buttonReadDocs": "Doku öffnen",
+
+  "settingsTitle": "Hub-Endpunkt",
+  "settingsHost": "Host",
+  "settingsPort": "Port",
+  "settingsTls": "TLS",
+  "settingsSaveAndConnect": "Speichern & verbinden",
+  "settingsLanguage": "Sprache",
+  "settingsLanguageHint": "Sprache der App. Wirkt auf alle Seiten; mehrsprachige Store-Einträge folgen ebenfalls.",
+
+  "channelsHeader": "KANÄLE",
+  "channelsActive": "aktiv",
+  "channelsRunning": "läuft",
+  "channelsStopped": "gestoppt",
+  "channelsConnect": "Studio mit diesem Kanal verbinden",
+  "channelsSwitch": "Diesen Kanal aktivieren",
+  "channelsEnableAutostart": "Autostart bei Anmeldung aktivieren",
+  "channelsDisableAutostart": "Autostart deaktivieren",
+
+  "storeSearchHint": "Module suchen — Name, Tagline, Tag, Capability…",
+  "storeFeatured": "EMPFOHLEN",
+  "storeFeaturedHint": "kuratierte Auswahl · klicken für Details",
+  "storeNoMatches": "Keine Treffer",
+  "storeNoMatchesHint": "Filter anpassen oder Suche leeren, um alle Einträge zu sehen.",
+  "storeClearFilters": "Filter zurücksetzen",
+  "storeStatusAll": "Alle",
+  "storeStatusPublished": "Veröffentlicht",
+  "storeStatusAlpha": "Alpha",
+  "storeStatusPlanned": "Geplant",
+  "storeInstalledOnly": "Installiert",
+  "storeNResults": "{n} Treffer",
+  "@storeNResults": {
+    "placeholders": { "n": { "type": "int" } }
+  },
+
+  "auditTitle": "Protokoll",
+  "auditNoEvents": "Noch keine Ereignisse",
+  "auditNoEventsHint": "Starte einen Flow, um den Audit-Stream zu füllen.\nEreignisse erscheinen innerhalb von Sekunden.",
+
+  "modulesTitle": "Module",
+  "modulesNoneTitle": "Noch keine Module",
+  "modulesRecentActivity": "LETZTE AKTIVITÄT",
+
+  "approvalsTitle": "Freigaben",
+  "approvalsTabPending": "Offen",
+  "approvalsTabHistory": "Verlauf",
+  "approvalsInboxZero": "Posteingang leer",
+  "approvalsInboxHint": "Alles erledigt. Neue `system.approval@^0`-Schritte landen automatisch hier.",
+
+  "doctorTitle": "Diagnose",
+
+  "mcpHeader": "MCP-CLIENTS",
+  "mcpHint": "Externe MCP-Server bringen ihre Tools als `mcp.<server>.<tool>` Capabilities ein. Einmal konfigurieren, im Store sichtbar.",
+  "mcpEmpty": "Keine MCP-Server konfiguriert. + klicken zum Hinzufügen.",
+  "n8nHeader": "N8N-ENDPUNKTE",
+  "n8nHint": "Aktive n8n-Workflows erscheinen als `n8n.<endpoint>.<slug>` Capabilities. Einmal konfigurieren, im Store sichtbar.",
+  "n8nEmpty": "Keine n8n-Endpunkte konfiguriert. + klicken zum Hinzufügen.",
+
+  "hubUnreachable": "Hub nicht erreichbar",
+  "hubUnreachableHint": "Hub starten mit `fai serve`.",
+
+  "languageEnglish": "English",
+  "languageGerman": "Deutsch"
+}