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_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}",