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/editor/persistence/sidecar_repository.dart

@@ -21,6 +21,7 @@
 import 'dart:async';
 import 'dart:io';
 
+import '../../models/bookmark.dart';
 import '../../models/scratch_link.dart';
 import '../../storage/badnote_sidecar.dart';
 import '../../storage/sidecar_store.dart';
@@ -139,6 +140,9 @@ class SidecarRepository {
   /// Scratch-link anchors loaded from the sidecar.
   List<SidecarScratchLink> get loadedScratchLinks => _sidecar.scratchLinks;
 
+  /// Bookmarks loaded from the sidecar.
+  List<Bookmark> get loadedBookmarks => _sidecar.bookmarks;
+
   /// The current in-memory sidecar (for tests / inspection).
   BadnoteSidecar get sidecar => _sidecar;
 
@@ -232,6 +236,30 @@ class SidecarRepository {
     _replace(scratchLinks: next);
   }
 
+  /// Add (or update, by id) a bookmark and schedule a save.
+  void scheduleBookmarkUpsert(Bookmark bookmark) {
+    final next = List<Bookmark>.of(_sidecar.bookmarks);
+    final idx = next.indexWhere((b) => b.id == bookmark.id);
+    if (idx == -1) {
+      next.add(bookmark);
+    } else {
+      next[idx] = bookmark;
+    }
+    _replace(bookmarks: next);
+  }
+
+  /// Remove the bookmark by [bookmarkId] and schedule a save.
+  void scheduleBookmarkDelete(String bookmarkId) {
+    final next =
+        _sidecar.bookmarks.where((b) => b.id != bookmarkId).toList();
+    _replace(bookmarks: next);
+  }
+
+  /// Replace the whole bookmark list and schedule a save.
+  void scheduleBookmarksSave(List<Bookmark> bookmarks) {
+    _replace(bookmarks: List<Bookmark>.of(bookmarks));
+  }
+
   /// The embedded scratchpad for [linkId], or null if the anchor is unknown.
   SidecarScratchpad? scratchpadFor(String linkId) {
     for (final s in _sidecar.scratchLinks) {
@@ -274,6 +302,7 @@ class SidecarRepository {
     String? title,
     Map<int, List<EditorStroke>>? strokes,
     Map<int, List<SidecarHighlight>>? highlights,
+    List<Bookmark>? bookmarks,
     List<SidecarScratchLink>? scratchLinks,
     String? ocrText,
     bool clearOcrText = false,
@@ -291,7 +320,7 @@ class SidecarRepository {
       updatedAt: DateTime.now().toUtc(),
       strokes: strokes ?? _sidecar.strokes,
       highlights: highlights ?? _sidecar.highlights,
-      bookmarks: _sidecar.bookmarks,
+      bookmarks: bookmarks ?? _sidecar.bookmarks,
       scratchLinks: scratchLinks ?? _sidecar.scratchLinks,
       ocrText: clearOcrText ? null : (ocrText ?? _sidecar.ocrText),
       background: background ?? _sidecar.background,