feat(board): sticky-note board with backlinks

Wire the F7 双链 + 无限便利贴 model (Board/LinkGraph) into a
reachable screen. Previously the model existed but had no UI and
no entry point.

board_screen.dart: an infinite InteractiveViewer canvas of
draggable, editable sticky cards. Card text renders [[links]] as
tappable chips that pan to the target card (dangling links styled
apart). A backlinks panel lists "linked from" via backlinksOf.
"Add card" FAB drops a card at the viewport center.

Persistence: a board_cards table (DB v7), one row per card,
debounced 800ms like the ink editors, loaded on open — boards
survive restart. Entry added to the home screen app bar
(dashboard_customize icon).

Ink-on-cards, multi-board management and link autocomplete are
deferred (TODO board-ink / board-multi / board-link-autocomplete).
analyze clean, 285 tests green.

lib/screens/home_screen.dart

@@ -11,6 +11,7 @@ import '../services/pdf_service.dart';
 import '../services/pptx_service.dart';
 import '../editor/canvas/pen_note_screen.dart';
 import '../editor/canvas/pen_slide_screen.dart';
+import 'board_screen.dart';
 import 'search_screen.dart';
 import 'settings_screen.dart';
 import 'split_view_screen.dart';
@@ -60,6 +61,15 @@ class HomeScreen extends ConsumerWidget {
             tooltip: l.importPpt,
             onPressed: () => _importPptx(context),
           ),
+          IconButton(
+            icon: const Icon(Icons.dashboard_customize),
+            tooltip: l.boardOpen,
+            onPressed: () {
+              Navigator.of(
+                context,
+              ).push(MaterialPageRoute(builder: (_) => const BoardScreen()));
+            },
+          ),
           IconButton(
             icon: const Icon(Icons.search),
             tooltip: l.search,