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/models/scratch_link.dart

@@ -0,0 +1,88 @@
+// lib/models/scratch_link.dart
+//
+// A PDF-anchored scratch link: a sticky-note "tab" placed at a normalized
+// position (nx, ny in [0,1]) on a specific page of a document. Tapping the
+// anchor opens a split view whose right pane is an infinite freehand scratchpad
+// that BELONGS TO THIS ANCHOR (keyed by [id]).
+//
+// Plain immutable value class (no freezed codegen) so it compiles without a
+// build_runner step. Equality is by value so anchors can be diffed in lists.
+
+import 'package:flutter/foundation.dart';
+
+@immutable
+class ScratchLink {
+  const ScratchLink({
+    required this.id,
+    required this.documentId,
+    required this.pageIndex,
+    required this.nx,
+    required this.ny,
+  });
+
+  /// Stable anchor id (uuid). Doubles as the scratchpad storage key so each
+  /// anchor gets its own private infinite scratchpad.
+  final String id;
+
+  /// The owning document (the editor's stable document-id for the PDF path).
+  final String documentId;
+
+  /// 0-based page the anchor sits on.
+  final int pageIndex;
+
+  /// Normalized horizontal position on the page, in [0, 1].
+  final double nx;
+
+  /// Normalized vertical position on the page, in [0, 1].
+  final double ny;
+
+  ScratchLink copyWith({
+    String? id,
+    String? documentId,
+    int? pageIndex,
+    double? nx,
+    double? ny,
+  }) =>
+      ScratchLink(
+        id: id ?? this.id,
+        documentId: documentId ?? this.documentId,
+        pageIndex: pageIndex ?? this.pageIndex,
+        nx: nx ?? this.nx,
+        ny: ny ?? this.ny,
+      );
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'documentId': documentId,
+        'pageIndex': pageIndex,
+        'nx': nx,
+        'ny': ny,
+      };
+
+  factory ScratchLink.fromJson(Map<String, dynamic> json) => ScratchLink(
+        id: json['id'] as String,
+        documentId: json['documentId'] as String,
+        pageIndex: (json['pageIndex'] as num).toInt(),
+        nx: (json['nx'] as num).toDouble(),
+        ny: (json['ny'] as num).toDouble(),
+      );
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is ScratchLink &&
+          runtimeType == other.runtimeType &&
+          id == other.id &&
+          documentId == other.documentId &&
+          pageIndex == other.pageIndex &&
+          nx == other.nx &&
+          ny == other.ny;
+
+  @override
+  int get hashCode => Object.hash(id, documentId, pageIndex, nx, ny);
+
+  @override
+  String toString() =>
+      'ScratchLink(id: $id, documentId: $documentId, pageIndex: $pageIndex, '
+      'nx: $nx, ny: $ny)';
+}