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_localizations_zh.dart

@@ -247,6 +247,12 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get toolPlaceScratchLink => '放置便签链接';
 
+  @override
+  String get toolText => '文字（点按或双击添加）';
+
+  @override
+  String get textPlaceholder => '输入文字…';
+
   @override
   String get scratchLinkDeleteTitle => '删除便签链接？';