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/l10n/app_localizations_en.dart

@@ -188,4 +188,39 @@ class AppLocalizationsEn extends AppLocalizations {
   String pageOfPages(int current, int total) {
     return '$current / $total';
   }
+
+  @override
+  String get board => 'Board';
+
+  @override
+  String get boardTitle => 'Sticky Board';
+
+  @override
+  String get boardOpen => 'Sticky note board';
+
+  @override
+  String get boardAddCard => 'Add card';
+
+  @override
+  String get boardNewCardText => 'New note';
+
+  @override
+  String get boardDeleteCard => 'Delete card';
+
+  @override
+  String get boardDeleteCardTitle => 'Delete this card?';
+
+  @override
+  String get boardBacklinks => 'Linked from';
+
+  @override
+  String get boardNoBacklinks => 'Nothing links here yet';
+
+  @override
+  String boardDanglingLink(String target) {
+    return 'No card named \"$target\"';
+  }
+
+  @override
+  String get close => 'Close';
 }