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_en.arb

@@ -93,6 +93,20 @@
   "toolPlaceScratchLink": "Place scratch link",
   "scratchLinkDeleteTitle": "Delete scratch link?",
   "scratchLinkDeleteBody": "This removes the anchor and its private scratchpad.",
+  "toolAddBookmark": "Add bookmark (here or at selection)",
+  "toolBookmarks": "Bookmarks",
+  "bookmarksTitle": "Bookmarks",
+  "bookmarksEmpty": "No bookmarks yet.",
+  "bookmarkDefaultLabel": "Page {page}",
+  "@bookmarkDefaultLabel": {
+    "placeholders": { "page": { "type": "int" } }
+  },
+  "bookmarkPageLabel": "Page {page}",
+  "@bookmarkPageLabel": {
+    "placeholders": { "page": { "type": "int" } }
+  },
+  "bookmarkDeleteTitle": "Delete bookmark?",
+  "bookmarkDeleteBody": "This removes the saved location.",
   "failedToOpenPdf": "Failed to open PDF:\n{error}",
   "@failedToOpenPdf": {
     "placeholders": { "error": { "type": "String" } }

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:

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';

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';

lib/l10n/app_zh.arb

@@ -78,6 +78,20 @@
   "toolPlaceScratchLink": "放置便签链接",
   "scratchLinkDeleteTitle": "删除便签链接？",
   "scratchLinkDeleteBody": "这会移除锚点及其专属草稿纸。",
+  "toolAddBookmark": "添加书签（当前位置或所选段落）",
+  "toolBookmarks": "书签",
+  "bookmarksTitle": "书签",
+  "bookmarksEmpty": "还没有书签。",
+  "bookmarkDefaultLabel": "第 {page} 页",
+  "@bookmarkDefaultLabel": {
+    "placeholders": { "page": { "type": "int" } }
+  },
+  "bookmarkPageLabel": "第 {page} 页",
+  "@bookmarkPageLabel": {
+    "placeholders": { "page": { "type": "int" } }
+  },
+  "bookmarkDeleteTitle": "删除书签？",
+  "bookmarkDeleteBody": "这会移除保存的位置。",
   "failedToOpenPdf": "打开 PDF 失败：\n{error}",
   "pdfNoPages": "PDF 没有任何页面。",
   "pageOfPages": "{current} / {total}",