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/l10n/app_en.arb

@@ -63,6 +63,9 @@
   "nextPage": "Next page",
   "toolSelectText": "Select text",
   "actionHighlightSelection": "Highlight selection",
+  "toolPlaceScratchLink": "Place scratch link",
+  "scratchLinkDeleteTitle": "Delete scratch link?",
+  "scratchLinkDeleteBody": "This removes the anchor and its private scratchpad.",
   "failedToOpenPdf": "Failed to open PDF:\n{error}",
   "@failedToOpenPdf": {
     "placeholders": { "error": { "type": "String" } }

lib/l10n/app_localizations.dart

@@ -422,6 +422,24 @@ abstract class AppLocalizations {
   /// **'Highlight selection'**
   String get actionHighlightSelection;
 
+  /// No description provided for @toolPlaceScratchLink.
+  ///
+  /// In en, this message translates to:
+  /// **'Place scratch link'**
+  String get toolPlaceScratchLink;
+
+  /// No description provided for @scratchLinkDeleteTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete scratch link?'**
+  String get scratchLinkDeleteTitle;
+
+  /// No description provided for @scratchLinkDeleteBody.
+  ///
+  /// In en, this message translates to:
+  /// **'This removes the anchor and its private scratchpad.'**
+  String get scratchLinkDeleteBody;
+
   /// No description provided for @failedToOpenPdf.
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_en.dart

@@ -176,6 +176,16 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get actionHighlightSelection => 'Highlight selection';
 
+  @override
+  String get toolPlaceScratchLink => 'Place scratch link';
+
+  @override
+  String get scratchLinkDeleteTitle => 'Delete scratch link?';
+
+  @override
+  String get scratchLinkDeleteBody =>
+      'This removes the anchor and its private scratchpad.';
+
   @override
   String failedToOpenPdf(String error) {
     return 'Failed to open PDF:\n$error';

lib/l10n/app_localizations_zh.dart

@@ -176,6 +176,15 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get actionHighlightSelection => '高亮所选';
 
+  @override
+  String get toolPlaceScratchLink => '放置便签链接';
+
+  @override
+  String get scratchLinkDeleteTitle => '删除便签链接？';
+
+  @override
+  String get scratchLinkDeleteBody => '这会移除锚点及其专属草稿纸。';
+
   @override
   String failedToOpenPdf(String error) {
     return '打开 PDF 失败：\n$error';

lib/l10n/app_zh.arb

@@ -54,6 +54,9 @@
   "nextPage": "下一页",
   "toolSelectText": "选择文字",
   "actionHighlightSelection": "高亮所选",
+  "toolPlaceScratchLink": "放置便签链接",
+  "scratchLinkDeleteTitle": "删除便签链接？",
+  "scratchLinkDeleteBody": "这会移除锚点及其专属草稿纸。",
   "failedToOpenPdf": "打开 PDF 失败：\n{error}",
   "pdfNoPages": "PDF 没有任何页面。",
   "pageOfPages": "{current} / {total}",