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_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';