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/board_screen.dart

@@ -134,6 +134,7 @@ class _BoardScreenState extends ConsumerState<BoardScreen> {
     final selected = _selectedId != null ? _board.cardById(_selectedId!) : null;
     final backlinks =
         selected != null ? _board.backlinksOf(selected.id) : <String>{};
+    final isEmpty = _board.length == 0;
 
     return Scaffold(
       backgroundColor: Theme.of(context).colorScheme.surface,
@@ -153,7 +154,43 @@ class _BoardScreenState extends ConsumerState<BoardScreen> {
             ),
         ],
       ),
-      body: Row(
+      body: isEmpty
+          ? Center(
+              child: Padding(
+                padding: const EdgeInsets.all(32),
+                child: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: [
+                    Icon(
+                      Icons.sticky_note_2_outlined,
+                      size: 72,
+                      color: AppTokens.copper.withValues(alpha: 0.85),
+                    ),
+                    const SizedBox(height: 20),
+                    Text(
+                      l.boardEmptyTitle,
+                      style: Theme.of(context).textTheme.headlineSmall,
+                      textAlign: TextAlign.center,
+                    ),
+                    const SizedBox(height: 8),
+                    Text(
+                      l.boardEmptyBody,
+                      style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                            color: AppTokens.inkMuted,
+                          ),
+                      textAlign: TextAlign.center,
+                    ),
+                    const SizedBox(height: 24),
+                    FilledButton.icon(
+                      onPressed: _addCard,
+                      icon: const Icon(Icons.add),
+                      label: Text(l.boardAddCard),
+                    ),
+                  ],
+                ),
+              ),
+            )
+          : Row(
         children: [
           Expanded(
             child: InteractiveViewer(
@@ -242,7 +279,7 @@ class _BoardScreenState extends ConsumerState<BoardScreen> {
                       ),
                       const SizedBox(height: 4),
                       Text(
-                        '在便利贴正文里写 [[另一张卡片id]] 建立双链',
+                        l.boardEmptyBody,
                         style: Theme.of(context).textTheme.bodySmall?.copyWith(
                               color: AppTokens.inkMuted,
                             ),