feat(pdf): paragraph-precise bookmarks

Add a bookmark tool to the PDF editor. A bookmark anchors to a precise
location: when text is selected it captures the selection's normalized
rect + the start char index in the page text (the true paragraph
anchor); with no selection it falls back to the tapped page + point.

- Bookmark model gains optional normalized anchor rect + charIndex +
  label (all absent from JSON when null, so old sidecars still load).
- Bookmarks persist in the sidecar (scheduleBookmarkUpsert) and load on
  open; a bookmarks panel lists them and tapping one jumps to its page.
  Delete is persisted.

Scoped to the PDF editor (note bookmarks later); scroll-to-anchor is
page-level for now. analyze clean, 391 tests green.

lib/models/bookmark.g.dart

@@ -14,6 +14,11 @@ _$BookmarkImpl _$$BookmarkImplFromJson(Map<String, dynamic> json) =>
       label: json['label'] as String? ?? '',
       color: (json['color'] as num?)?.toInt() ?? 0xFF2196F3,
       createdAt: DateTime.parse(json['createdAt'] as String),
+      anchorLeft: (json['anchorLeft'] as num?)?.toDouble(),
+      anchorTop: (json['anchorTop'] as num?)?.toDouble(),
+      anchorRight: (json['anchorRight'] as num?)?.toDouble(),
+      anchorBottom: (json['anchorBottom'] as num?)?.toDouble(),
+      charIndex: (json['charIndex'] as num?)?.toInt(),
     );
 
 Map<String, dynamic> _$$BookmarkImplToJson(_$BookmarkImpl instance) =>
@@ -24,4 +29,9 @@ Map<String, dynamic> _$$BookmarkImplToJson(_$BookmarkImpl instance) =>
       'label': instance.label,
       'color': instance.color,
       'createdAt': instance.createdAt.toIso8601String(),
+      'anchorLeft': instance.anchorLeft,
+      'anchorTop': instance.anchorTop,
+      'anchorRight': instance.anchorRight,
+      'anchorBottom': instance.anchorBottom,
+      'charIndex': instance.charIndex,
     };