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"
 }