feat(pdf): typed-text tool (Windows-Ink friendly)

Add a text-annotation tool to the PDF editor. With the text tool a
pen-tap, or a mouse double-click, drops a text box at that normalized
page point and focuses a real Flutter TextField — so the OS IME and the
Windows-Ink handwriting panel feed it (device-validated). Tapping an
existing box re-opens it; clearing it deletes it.

- SidecarText {nx, ny, text, fontSize (page-relative), color} per page,
  glued under zoom; stored in the sidecar `texts` field (back-compat
  missing -> none), saved via scheduleTextsSave and loaded on open.
- Rendered in pageOverlaysBuilder at the scaled position.

PDF editor only for now (note text later). analyze clean, 397 tests.

lib/l10n/app_zh.arb

@@ -76,6 +76,8 @@
   "actionHighlightSelection": "高亮所选",
   "toolRemoveHighlight": "移除高亮（点按高亮处）",
   "toolPlaceScratchLink": "放置便签链接",
+  "toolText": "文字（点按或双击添加）",
+  "textPlaceholder": "输入文字…",
   "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）",