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_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}",