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_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: