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.dart

@@ -554,6 +554,18 @@ abstract class AppLocalizations {
   /// **'Place scratch link'**
   String get toolPlaceScratchLink;
 
+  /// No description provided for @toolText.
+  ///
+  /// In en, this message translates to:
+  /// **'Text (tap or double-click to add)'**
+  String get toolText;
+
+  /// No description provided for @textPlaceholder.
+  ///
+  /// In en, this message translates to:
+  /// **'Type…'**
+  String get textPlaceholder;
+
   /// No description provided for @scratchLinkDeleteTitle.
   ///
   /// In en, this message translates to: