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.dart

@@ -439,6 +439,72 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'{current} / {total}'**
   String pageOfPages(int current, int total);
+
+  /// No description provided for @board.
+  ///
+  /// In en, this message translates to:
+  /// **'Board'**
+  String get board;
+
+  /// No description provided for @boardTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Sticky Board'**
+  String get boardTitle;
+
+  /// No description provided for @boardOpen.
+  ///
+  /// In en, this message translates to:
+  /// **'Sticky note board'**
+  String get boardOpen;
+
+  /// No description provided for @boardAddCard.
+  ///
+  /// In en, this message translates to:
+  /// **'Add card'**
+  String get boardAddCard;
+
+  /// No description provided for @boardNewCardText.
+  ///
+  /// In en, this message translates to:
+  /// **'New note'**
+  String get boardNewCardText;
+
+  /// No description provided for @boardDeleteCard.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete card'**
+  String get boardDeleteCard;
+
+  /// No description provided for @boardDeleteCardTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete this card?'**
+  String get boardDeleteCardTitle;
+
+  /// No description provided for @boardBacklinks.
+  ///
+  /// In en, this message translates to:
+  /// **'Linked from'**
+  String get boardBacklinks;
+
+  /// No description provided for @boardNoBacklinks.
+  ///
+  /// In en, this message translates to:
+  /// **'Nothing links here yet'**
+  String get boardNoBacklinks;
+
+  /// No description provided for @boardDanglingLink.
+  ///
+  /// In en, this message translates to:
+  /// **'No card named \"{target}\"'**
+  String boardDanglingLink(String target);
+
+  /// No description provided for @close.
+  ///
+  /// In en, this message translates to:
+  /// **'Close'**
+  String get close;
 }
 
 class _AppLocalizationsDelegate