feat(pdf): anchored scratch links replace board

Replace the rejected standalone sticky-card board with the real
feature: place a link anchor anywhere on a PDF page, tap it to open
split view whose right pane is THAT anchor's own infinite scratchpad
(keyed by anchor id) — like a paper sticky-note tab.

- ScratchLink model + scratch_links table (id, doc, page, nx, ny).
- PDF editor: "place link" tool drops/loads/shows tappable markers;
  tap opens SplitViewScreen for that anchor; long-press deletes.
- SplitViewScreen rebuilt on pdfrx (was syncfusion), right scratchpad
  keyed by scratchLinkId, new brush palette (was AnnotationToolbar).
- Remove board_screen + its test + the home board entry.

analyze clean, tests green.

lib/services/database_service.dart

@@ -16,6 +16,7 @@ import '../models/ink_stroke.dart';
 import '../models/note.dart';
 import '../models/pen_tool.dart';
 import '../models/pointer_device_kind.dart';
+import '../models/scratch_link.dart';
 
 class DatabaseService {
   static DatabaseService? _instance;
@@ -56,7 +57,7 @@ class DatabaseService {
 
     _database = await openDatabase(
       dbPath,
-      version: 7,
+      version: 8,
       onCreate: _onCreate,
       onUpgrade: _onUpgrade,
     );
@@ -202,6 +203,28 @@ class DatabaseService {
 
     // Sticky-note board cards (v7): F7 双链 + 无限便利贴.
     await _createBoardCardsTable(db);
+
+    // PDF-anchored scratch links (v8).
+    await _createScratchLinksTable(db);
+  }
+
+  /// PDF-anchored scratch links table (v8). One row per [ScratchLink] anchor.
+  /// The anchor [id] doubles as the storage key for its private scratchpad
+  /// (reused from the [scratchpads] table — see [saveScratchpad]).
+  Future<void> _createScratchLinksTable(DatabaseExecutor db) async {
+    await db.execute('''
+      CREATE TABLE scratch_links (
+        id TEXT PRIMARY KEY,
+        document_id TEXT NOT NULL,
+        page_index INTEGER NOT NULL,
+        nx REAL NOT NULL,
+        ny REAL NOT NULL,
+        created_at TEXT NOT NULL
+      )
+    ''');
+    await db.execute(
+      'CREATE INDEX idx_scratch_links_doc ON scratch_links(document_id)',
+    );
   }
 
   /// Sticky-note board cards table (F7). One row per [BoardCard]; a board is the
@@ -232,6 +255,13 @@ class DatabaseService {
     if (oldVersion < 5) await _migrateV4toV5(db);
     if (oldVersion < 6) await _migrateV5toV6(db);
     if (oldVersion < 7) await _migrateV6toV7(db);
+    if (oldVersion < 8) await _migrateV7toV8(db);
+  }
+
+  Future<void> _migrateV7toV8(Database db) async {
+    await db.transaction((txn) async {
+      await _createScratchLinksTable(txn);
+    });
   }
 
   Future<void> _migrateV6toV7(Database db) async {
@@ -1003,4 +1033,54 @@ class DatabaseService {
         ),
     ]);
   }
+
+  // ── Scratch links CRUD (PDF-anchored scratchpad tabs) ──────────────────
+
+  /// Insert or replace a [ScratchLink] anchor. The anchor's private scratchpad
+  /// lives in the [scratchpads] table keyed by [ScratchLink.id] — saved/loaded
+  /// via [saveScratchpad] / [loadScratchpad].
+  Future<void> saveScratchLink(ScratchLink link) async {
+    await _database.insert(
+      'scratch_links',
+      {
+        'id': link.id,
+        'document_id': link.documentId,
+        'page_index': link.pageIndex,
+        'nx': link.nx,
+        'ny': link.ny,
+        'created_at': DateTime.now().toIso8601String(),
+      },
+      conflictAlgorithm: ConflictAlgorithm.replace,
+    );
+  }
+
+  /// Load all anchors for [documentId], oldest first.
+  Future<List<ScratchLink>> loadScratchLinks(String documentId) async {
+    final rows = await _database.query(
+      'scratch_links',
+      where: 'document_id = ?',
+      whereArgs: [documentId],
+      orderBy: 'created_at ASC',
+    );
+    return rows
+        .map(
+          (row) => ScratchLink(
+            id: row['id'] as String,
+            documentId: row['document_id'] as String,
+            pageIndex: row['page_index'] as int,
+            nx: (row['nx'] as num).toDouble(),
+            ny: (row['ny'] as num).toDouble(),
+          ),
+        )
+        .toList();
+  }
+
+  /// Delete an anchor and its private scratchpad (the scratchpad row keyed by
+  /// the anchor id), so a deleted anchor leaves no orphaned ink behind.
+  Future<void> deleteScratchLink(String id) async {
+    await _database.transaction((txn) async {
+      await txn.delete('scratch_links', where: 'id = ?', whereArgs: [id]);
+      await txn.delete('scratchpads', where: 'document_id = ?', whereArgs: [id]);
+    });
+  }
 }