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_en.arb

@@ -0,0 +1,94 @@
+{
+  "@@locale": "en",
+
+  "appTitle": "F∆I Studio",
+  "@appTitle": { "description": "App-bar title in the OS task switcher." },
+
+  "navDoctor": "Doctor",
+  "navModules": "Modules",
+  "navStore": "Store",
+  "navFlows": "Flows",
+  "navAudit": "Audit",
+  "navApprovals": "Approvals",
+
+  "connectionConnected": "connected",
+  "connectionUnreachable": "unreachable",
+  "connectionConnecting": "connecting…",
+  "connectionStartHub": "Start hub",
+
+  "buttonCancel": "Cancel",
+  "buttonSave": "Save",
+  "buttonClose": "Close",
+  "buttonRetry": "Retry",
+  "buttonRefresh": "Refresh",
+  "buttonInstall": "Install",
+  "buttonUninstall": "Uninstall",
+  "buttonDetails": "Details",
+  "buttonAdd": "Add",
+  "buttonRemove": "Remove",
+  "buttonOpen": "Open",
+  "buttonExplain": "Explain",
+  "buttonReadDocs": "Read docs",
+
+  "settingsTitle": "Hub endpoint",
+  "settingsHost": "Host",
+  "settingsPort": "Port",
+  "settingsTls": "TLS",
+  "settingsSaveAndConnect": "Save & connect",
+  "settingsLanguage": "Language",
+  "settingsLanguageHint": "App language. Affects every page; bilingual store entries also flip via this toggle.",
+
+  "channelsHeader": "CHANNELS",
+  "channelsActive": "active",
+  "channelsRunning": "running",
+  "channelsStopped": "stopped",
+  "channelsConnect": "Connect Studio to this channel",
+  "channelsSwitch": "Make this the active channel",
+  "channelsEnableAutostart": "Enable autostart at login",
+  "channelsDisableAutostart": "Disable autostart",
+
+  "storeSearchHint": "Search modules — name, tagline, tag, capability…",
+  "storeFeatured": "FEATURED",
+  "storeFeaturedHint": "curated picks · click for details",
+  "storeNoMatches": "No matches",
+  "storeNoMatchesHint": "Adjust the filters or clear the search to see all entries.",
+  "storeClearFilters": "Clear filters",
+  "storeStatusAll": "All",
+  "storeStatusPublished": "Published",
+  "storeStatusAlpha": "Alpha",
+  "storeStatusPlanned": "Planned",
+  "storeInstalledOnly": "Installed",
+  "storeNResults": "{n} result{n, plural, =1{} other{s}}",
+  "@storeNResults": {
+    "placeholders": { "n": { "type": "int" } }
+  },
+
+  "auditTitle": "Audit",
+  "auditNoEvents": "No events yet",
+  "auditNoEventsHint": "Run a flow to populate the audit stream.\nEvents appear here within seconds.",
+
+  "modulesTitle": "Modules",
+  "modulesNoneTitle": "No modules yet",
+  "modulesRecentActivity": "RECENT ACTIVITY",
+
+  "approvalsTitle": "Approvals",
+  "approvalsTabPending": "Pending",
+  "approvalsTabHistory": "History",
+  "approvalsInboxZero": "Inbox zero",
+  "approvalsInboxHint": "All caught up. New `system.approval@^0` steps land here automatically.",
+
+  "doctorTitle": "Doctor",
+
+  "mcpHeader": "MCP CLIENTS",
+  "mcpHint": "External MCP servers federate their tools as `mcp.<server>.<tool>` capabilities. Configure once, see them in the Store.",
+  "mcpEmpty": "No MCP servers configured. Click + to add one.",
+  "n8nHeader": "N8N ENDPOINTS",
+  "n8nHint": "Active n8n workflows surface as `n8n.<endpoint>.<slug>` capabilities. Configure once, see them in the Store.",
+  "n8nEmpty": "No n8n endpoints configured. Click + to add one.",
+
+  "hubUnreachable": "Hub unreachable",
+  "hubUnreachableHint": "Start the hub with `fai serve`.",
+
+  "languageEnglish": "English",
+  "languageGerman": "Deutsch"
+}