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.",

lib/l10n/app_localizations.dart

@@ -470,6 +470,12 @@ abstract class AppLocalizations {
   /// **'Highlight selection'**
   String get actionHighlightSelection;
 
+  /// No description provided for @toolRemoveHighlight.
+  ///
+  /// In en, this message translates to:
+  /// **'Remove highlight (tap a highlight)'**
+  String get toolRemoveHighlight;
+
   /// No description provided for @toolPlaceScratchLink.
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_en.dart

@@ -200,6 +200,9 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get actionHighlightSelection => 'Highlight selection';
 
+  @override
+  String get toolRemoveHighlight => 'Remove highlight (tap a highlight)';
+
   @override
   String get toolPlaceScratchLink => 'Place scratch link';
 

lib/l10n/app_localizations_zh.dart

@@ -200,6 +200,9 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get actionHighlightSelection => '高亮所选';
 
+  @override
+  String get toolRemoveHighlight => '移除高亮（点按高亮处）';
+
   @override
   String get toolPlaceScratchLink => '放置便签链接';
 

lib/l10n/app_zh.arb

@@ -62,6 +62,7 @@
   "nextPage": "下一页",
   "toolSelectText": "选择文字",
   "actionHighlightSelection": "高亮所选",
+  "toolRemoveHighlight": "移除高亮（点按高亮处）",
   "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。",