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.

test/badnote_sidecar_test.dart

@@ -219,6 +219,65 @@ void main() {
     expect(reparsed.version, kBadnoteSidecarVersion);
   });
 
+  test('paragraph-anchored bookmark round-trips its anchor + char index', () {
+    final original = BadnoteSidecar(bookmarks: [
+      Bookmark(
+        id: 'bm-anchor',
+        documentId: 'doc1',
+        pageNumber: 3,
+        label: 'A bookmarked paragraph',
+        color: 0xFF2196F3,
+        createdAt: DateTime.utc(2026, 6, 24, 11, 0, 0),
+        anchorLeft: 0.12,
+        anchorTop: 0.20,
+        anchorRight: 0.88,
+        anchorBottom: 0.235,
+        charIndex: 1234,
+      ),
+    ]);
+    final reparsed = _roundTrip(original);
+    expect(reparsed.bookmarks, original.bookmarks);
+    final bm = reparsed.bookmarks.single;
+    expect(bm.pageNumber, 3);
+    expect(bm.anchorLeft, 0.12);
+    expect(bm.anchorTop, 0.20);
+    expect(bm.anchorRight, 0.88);
+    expect(bm.anchorBottom, 0.235);
+    expect(bm.charIndex, 1234);
+  });
+
+  test('legacy page-only bookmark JSON decodes (anchor fields absent)', () {
+    // A bookmark authored before the anchor fields existed: only page-level.
+    final legacyJson = {
+      'badnoteSidecarVersion': 1,
+      'bookmarks': [
+        {
+          'id': 'legacy1',
+          'documentId': 'doc1',
+          'pageNumber': 7,
+          'label': 'Old bookmark',
+          'color': 0xFF2196F3,
+          'createdAt': '2026-06-01T00:00:00.000Z',
+        }
+      ],
+    };
+    final decoded = BadnoteSidecar.fromJson(legacyJson);
+    final bm = decoded.bookmarks.single;
+    expect(bm.pageNumber, 7);
+    expect(bm.anchorLeft, isNull);
+    expect(bm.anchorTop, isNull);
+    expect(bm.anchorRight, isNull);
+    expect(bm.anchorBottom, isNull);
+    expect(bm.charIndex, isNull);
+    // Re-encoding preserves the page-only data; any anchor keys are null.
+    final reJson = bm.toJson();
+    expect(reJson['anchorLeft'], isNull);
+    expect(reJson['charIndex'], isNull);
+    expect(reJson['pageNumber'], 7);
+    // And it decodes back to an equal page-only bookmark.
+    expect(Bookmark.fromJson(reJson), bm);
+  });
+
   test('unknown fields are ignored (forward-compat)', () {
     final encoded = jsonEncode(BadnoteSidecar(
       strokes: {0: [_editorStroke('s0', EditorTool.pen)]},