feat(storage): PDF editor persists to per-file sidecar

Phase 2 (core swap). The PDF editor and split-view scratchpad stop
writing SQLite and persist to a per-file sidecar
`<pdfPath>.badnote.json` (debounced, atomic temp+rename+.bak) — so
annotations travel with the file. The source path is the identity
(no more djb2 doc-id).

- SidecarRepository wraps the Phase-1 store with debounced autosave.
- pen_editor: per-page ink, scratch-links AND highlights now persist
  to the sidecar and restore on reopen (closes persist-highlights).
- New "un-highlight" tool: tap a stored highlight to remove it — the
  highlight could not be removed before.
- split_view: each anchor's scratchpad lives in the sidecar's
  scratchLinks[id].scratchpad, keyed by anchor id.

Note: pre-existing SQLite annotations are migrated later (Phase 5);
note/slide editors swap in Phase 4. analyze clean, tests green.

lib/l10n/app_en.arb

@@ -71,6 +71,7 @@
   "nextPage": "Next page",
   "toolSelectText": "Select text",
   "actionHighlightSelection": "Highlight selection",
+  "toolRemoveHighlight": "Remove highlight (tap a highlight)",
   "toolPlaceScratchLink": "Place scratch link",
   "scratchLinkDeleteTitle": "Delete scratch link?",
   "scratchLinkDeleteBody": "This removes the anchor and its private scratchpad.",