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_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"
 }