feat: vault-aligned server v1 + UX polish

Redesign the optional FastAPI companion around vault files (manifest /
PUT/GET/DELETE + OCR jobs) instead of legacy strokes_json notes. Wire a
client Server settings panel for health/login. Polish shell UX: l10n for
settings/home/board, sticky-board empty state, and a narrow-screen
diagnostics FAB.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/screens/app_shell.dart

@@ -115,18 +115,29 @@ class _AppShellState extends ConsumerState<AppShell> {
 
     return Scaffold(
       body: body,
-      bottomNavigationBar: NavigationBar(
-        selectedIndex: _index,
-        onDestinationSelected: _select,
-        destinations: [
-          for (final d in destinations)
-            NavigationDestination(
-              icon: Icon(d.icon),
-              selectedIcon: Icon(d.selectedIcon),
-              label: d.label,
-            ),
+      bottomNavigationBar: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          NavigationBar(
+            selectedIndex: _index,
+            onDestinationSelected: _select,
+            destinations: [
+              for (final d in destinations)
+                NavigationDestination(
+                  icon: Icon(d.icon),
+                  selectedIcon: Icon(d.selectedIcon),
+                  label: d.label,
+                ),
+            ],
+          ),
         ],
       ),
+      floatingActionButton: FloatingActionButton.small(
+        heroTag: 'diag_fab',
+        tooltip: AppLocalizations.of(context).diagnosticsSection,
+        onPressed: () => _diagKey.currentState?.toggle(),
+        child: const Icon(Icons.bug_report_outlined),
+      ),
     );
   }
 
@@ -150,14 +161,14 @@ Future<void> exportDiagnosticPack(BuildContext context) async {
     if (!context.mounted) return;
     ScaffoldMessenger.of(context).showSnackBar(
       SnackBar(
-        content: Text('诊断包: ${result.zipPath}'),
+        content: Text(AppLocalizations.of(context).diagExported(result.bytes)),
         duration: const Duration(seconds: 5),
       ),
     );
   } catch (e) {
     if (!context.mounted) return;
     ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text('导出失败: $e')),
+      SnackBar(content: Text(AppLocalizations.of(context).diagExportFail('$e'))),
     );
   }
 }