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_en.arb

@@ -74,5 +74,19 @@
       "current": { "type": "int" },
       "total": { "type": "int" }
     }
-  }
+  },
+  "board": "Board",
+  "boardTitle": "Sticky Board",
+  "boardOpen": "Sticky note board",
+  "boardAddCard": "Add card",
+  "boardNewCardText": "New note",
+  "boardDeleteCard": "Delete card",
+  "boardDeleteCardTitle": "Delete this card?",
+  "boardBacklinks": "Linked from",
+  "boardNoBacklinks": "Nothing links here yet",
+  "boardDanglingLink": "No card named \"{target}\"",
+  "@boardDanglingLink": {
+    "placeholders": { "target": { "type": "String" } }
+  },
+  "close": "Close"
 }

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

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';
 }

lib/l10n/app_localizations_zh.dart

@@ -188,4 +188,39 @@ class AppLocalizationsZh extends AppLocalizations {
   String pageOfPages(int current, int total) {
     return '$current / $total';
   }
+
+  @override
+  String get board => '便利贴板';
+
+  @override
+  String get boardTitle => '便利贴板';
+
+  @override
+  String get boardOpen => '便利贴板';
+
+  @override
+  String get boardAddCard => '添加便利贴';
+
+  @override
+  String get boardNewCardText => '新便利贴';
+
+  @override
+  String get boardDeleteCard => '删除便利贴';
+
+  @override
+  String get boardDeleteCardTitle => '删除这张便利贴？';
+
+  @override
+  String get boardBacklinks => '哪些链接到这里';
+
+  @override
+  String get boardNoBacklinks => '暂无其他便利贴链接到这里';
+
+  @override
+  String boardDanglingLink(String target) {
+    return '没有名为“$target”的便利贴';
+  }
+
+  @override
+  String get close => '关闭';
 }

lib/l10n/app_zh.arb

@@ -56,5 +56,16 @@
   "actionHighlightSelection": "高亮所选",
   "failedToOpenPdf": "打开 PDF 失败：\n{error}",
   "pdfNoPages": "PDF 没有任何页面。",
-  "pageOfPages": "{current} / {total}"
+  "pageOfPages": "{current} / {total}",
+  "board": "便利贴板",
+  "boardTitle": "便利贴板",
+  "boardOpen": "便利贴板",
+  "boardAddCard": "添加便利贴",
+  "boardNewCardText": "新便利贴",
+  "boardDeleteCard": "删除便利贴",
+  "boardDeleteCardTitle": "删除这张便利贴？",
+  "boardBacklinks": "哪些链接到这里",
+  "boardNoBacklinks": "暂无其他便利贴链接到这里",
+  "boardDanglingLink": "没有名为“{target}”的便利贴",
+  "close": "关闭"
 }