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" } }