feat(studio): first-run UX, recovery affordances, l10n, bundled fonts

- Connection-aware Welcome: when the hub is down, show a hero with a
  primary "Start hub" CTA + install fallback instead of a dead,
  all-unchecked onboarding checklist (the first-run cliff).
- Actionable binary-not-found (file picker + install link, not a
  "set FAI_BIN" dead end) and a connect-failure banner after
  repeated failed health polls.
- Localize six hardcoded English error/toast clusters (DE+EN ARB).
- Bundle Inter + JetBrains Mono as assets; drop the runtime
  google_fonts fetch (air-gap / KRITIS safe, no font-swap flash).

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

lib/widgets/fai_settings_dialog.dart

@@ -293,11 +293,12 @@ class _FaiSettingsDialogState extends State<FaiSettingsDialog> {
     });
     final r = await SystemActions.faiChannelSwitch(name);
     if (!mounted) return;
+    final l = AppLocalizations.of(context)!;
     setState(() {
       _saving = false;
       _channelToast = r.ok
-          ? 'Switched active channel to "$name". Daemon restarted.\n${r.stdout.trim()}'
-          : 'Channel switch failed: ${r.stderr.isEmpty ? r.stdout : r.stderr}';
+          ? '${l.channelSwitchOk(name)}\n${r.stdout.trim()}'
+          : l.channelSwitchFailed(r.stderr.isEmpty ? r.stdout : r.stderr);
     });
     if (r.ok) await _loadChannels();
   }