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/editor/persistence/sidecar_repository.dart

@@ -137,6 +137,9 @@ class SidecarRepository {
   /// Page index → highlight rects loaded from the sidecar.
   Map<int, List<SidecarHighlight>> get loadedHighlights => _sidecar.highlights;
 
+  /// Page index → typed-text annotations loaded from the sidecar.
+  Map<int, List<SidecarText>> get loadedTexts => _sidecar.texts;
+
   /// Scratch-link anchors loaded from the sidecar.
   List<SidecarScratchLink> get loadedScratchLinks => _sidecar.scratchLinks;
 
@@ -202,6 +205,17 @@ class SidecarRepository {
     _replace(highlights: next);
   }
 
+  /// Replace the typed-text annotations for [pageIndex] and schedule a save.
+  void scheduleTextsSave(int pageIndex, List<SidecarText> texts) {
+    final next = Map<int, List<SidecarText>>.from(_sidecar.texts);
+    if (texts.isEmpty) {
+      next.remove(pageIndex);
+    } else {
+      next[pageIndex] = List<SidecarText>.of(texts);
+    }
+    _replace(texts: next);
+  }
+
   /// Add (or update) a scratch-link anchor, preserving any existing scratchpad,
   /// and schedule a save.
   void scheduleScratchLinkUpsert(ScratchLink link) {
@@ -302,6 +316,7 @@ class SidecarRepository {
     String? title,
     Map<int, List<EditorStroke>>? strokes,
     Map<int, List<SidecarHighlight>>? highlights,
+    Map<int, List<SidecarText>>? texts,
     List<Bookmark>? bookmarks,
     List<SidecarScratchLink>? scratchLinks,
     String? ocrText,
@@ -320,6 +335,7 @@ class SidecarRepository {
       updatedAt: DateTime.now().toUtc(),
       strokes: strokes ?? _sidecar.strokes,
       highlights: highlights ?? _sidecar.highlights,
+      texts: texts ?? _sidecar.texts,
       bookmarks: bookmarks ?? _sidecar.bookmarks,
       scratchLinks: scratchLinks ?? _sidecar.scratchLinks,
       ocrText: clearOcrText ? null : (ocrText ?? _sidecar.ocrText),