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_en.dart

@@ -254,6 +254,34 @@ class AppLocalizationsEn extends AppLocalizations {
   String get scratchLinkDeleteBody =>
       'This removes the anchor and its private scratchpad.';
 
+  @override
+  String get toolAddBookmark => 'Add bookmark (here or at selection)';
+
+  @override
+  String get toolBookmarks => 'Bookmarks';
+
+  @override
+  String get bookmarksTitle => 'Bookmarks';
+
+  @override
+  String get bookmarksEmpty => 'No bookmarks yet.';
+
+  @override
+  String bookmarkDefaultLabel(int page) {
+    return 'Page $page';
+  }
+
+  @override
+  String bookmarkPageLabel(int page) {
+    return 'Page $page';
+  }
+
+  @override
+  String get bookmarkDeleteTitle => 'Delete bookmark?';
+
+  @override
+  String get bookmarkDeleteBody => 'This removes the saved location.';
+
   @override
   String failedToOpenPdf(String error) {
     return 'Failed to open PDF:\n$error';