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/l10n/app_localizations.dart

@@ -566,6 +566,54 @@ abstract class AppLocalizations {
   /// **'This removes the anchor and its private scratchpad.'**
   String get scratchLinkDeleteBody;
 
+  /// No description provided for @toolAddBookmark.
+  ///
+  /// In en, this message translates to:
+  /// **'Add bookmark (here or at selection)'**
+  String get toolAddBookmark;
+
+  /// No description provided for @toolBookmarks.
+  ///
+  /// In en, this message translates to:
+  /// **'Bookmarks'**
+  String get toolBookmarks;
+
+  /// No description provided for @bookmarksTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Bookmarks'**
+  String get bookmarksTitle;
+
+  /// No description provided for @bookmarksEmpty.
+  ///
+  /// In en, this message translates to:
+  /// **'No bookmarks yet.'**
+  String get bookmarksEmpty;
+
+  /// No description provided for @bookmarkDefaultLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Page {page}'**
+  String bookmarkDefaultLabel(int page);
+
+  /// No description provided for @bookmarkPageLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Page {page}'**
+  String bookmarkPageLabel(int page);
+
+  /// No description provided for @bookmarkDeleteTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete bookmark?'**
+  String get bookmarkDeleteTitle;
+
+  /// No description provided for @bookmarkDeleteBody.
+  ///
+  /// In en, this message translates to:
+  /// **'This removes the saved location.'**
+  String get bookmarkDeleteBody;
+
   /// No description provided for @failedToOpenPdf.
   ///
   /// In en, this message translates to: