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/models/bookmark.dart

@@ -3,15 +3,50 @@ import 'package:freezed_annotation/freezed_annotation.dart';
 part 'bookmark.freezed.dart';
 part 'bookmark.g.dart';
 
+/// A saved location in a document.
+///
+/// "Paragraph precision" (user ask: 精确到段落加书签) is expressed by the optional
+/// in-page anchor fields below, all normalized to the page in [0,1]:
+///
+///   * [anchorLeft]/[anchorTop]/[anchorRight]/[anchorBottom] — the bounding rect
+///     of the bookmarked text fragment (the FIRST fragment of the current text
+///     selection), in NORMALIZED page coords with a top-left origin (the same
+///     convention `SidecarHighlight` and the editor's highlight rects use). This
+///     is what jump-to scrolls to (via `goToRectInsidePage`), so the bookmark
+///     lands on the exact paragraph, not just the page top.
+///   * [charIndex] — the character index of the selection start in the page's
+///     `fullText` (the true text-position anchor). Stored for fidelity / future
+///     reflow-tolerant re-anchoring; not currently used for navigation.
+///
+/// When no text was selected the anchor falls back to the tapped point: only
+/// [anchorTop]/[anchorLeft] are set (a zero-size rect) and [charIndex] is null.
+/// All anchor fields are optional and absent from JSON when null, so OLD
+/// bookmarks (page-only) still decode and re-encode unchanged (back-compat).
 @freezed
 abstract class Bookmark with _$Bookmark {
   const factory Bookmark({
     required String id,
     required String documentId,
+
+    /// 1-based page number this bookmark lives on.
     required int pageNumber,
     @Default('') String label,
     @Default(0xFF2196F3) int color,
     required DateTime createdAt,
+
+    /// Normalized in-page anchor rect (top-left origin, [0,1]). Null for legacy
+    /// page-only bookmarks (and tap-fallback bookmarks set only top/left). Old
+    /// page-only bookmark JSON omits these keys; they decode to null and the
+    /// data round-trips (back-compat). A re-encoded legacy bookmark gains
+    /// explicit null keys, which readers tolerate.
+    double? anchorLeft,
+    double? anchorTop,
+    double? anchorRight,
+    double? anchorBottom,
+
+    /// Character index of the selection start in the page's `fullText`, or null
+    /// (tap-fallback / legacy bookmarks).
+    int? charIndex,
   }) = _Bookmark;
 
   factory Bookmark.fromJson(Map<String, dynamic> json) =>