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_zh.dart

@@ -253,6 +253,34 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get scratchLinkDeleteBody => '这会移除锚点及其专属草稿纸。';
 
+  @override
+  String get toolAddBookmark => '添加书签（当前位置或所选段落）';
+
+  @override
+  String get toolBookmarks => '书签';
+
+  @override
+  String get bookmarksTitle => '书签';
+
+  @override
+  String get bookmarksEmpty => '还没有书签。';
+
+  @override
+  String bookmarkDefaultLabel(int page) {
+    return '第 $page 页';
+  }
+
+  @override
+  String bookmarkPageLabel(int page) {
+    return '第 $page 页';
+  }
+
+  @override
+  String get bookmarkDeleteTitle => '删除书签？';
+
+  @override
+  String get bookmarkDeleteBody => '这会移除保存的位置。';
+
   @override
   String failedToOpenPdf(String error) {
     return '打开 PDF 失败：\n$error';