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" } }