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/storage/badnote_sidecar.dart

@@ -108,6 +108,95 @@ class SidecarHighlight {
       'SidecarHighlight(l: $l, t: $t, r: $r, b: $b, color: $color)';
 }
 
+/// A single typed-text annotation on a page (PDF editor for now). Position is
+/// NORMALIZED to the page rect ([nx],[ny] in [0,1]) so the box stays glued under
+/// zoom/scroll, exactly like [SidecarHighlight] / [ScratchLink]. [fontSize] is
+/// PAGE-RELATIVE (a fraction of the page width), so the rendered text scales
+/// with the page; the editor multiplies it by the on-screen page width.
+class SidecarText {
+  const SidecarText({
+    required this.id,
+    required this.nx,
+    required this.ny,
+    required this.text,
+    this.fontSize = 0.03,
+    this.color = 0xFF000000,
+  });
+
+  /// Stable id (uuid) so edits/deletes address a specific box.
+  final String id;
+
+  /// Normalized x of the box's top-left in [0,1].
+  final double nx;
+
+  /// Normalized y of the box's top-left in [0,1].
+  final double ny;
+
+  /// The typed text.
+  final String text;
+
+  /// Font size as a fraction of page WIDTH (page-relative; scales with zoom).
+  final double fontSize;
+
+  /// ARGB text color.
+  final int color;
+
+  SidecarText copyWith({
+    String? id,
+    double? nx,
+    double? ny,
+    String? text,
+    double? fontSize,
+    int? color,
+  }) =>
+      SidecarText(
+        id: id ?? this.id,
+        nx: nx ?? this.nx,
+        ny: ny ?? this.ny,
+        text: text ?? this.text,
+        fontSize: fontSize ?? this.fontSize,
+        color: color ?? this.color,
+      );
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'nx': nx,
+        'ny': ny,
+        'text': text,
+        'fontSize': fontSize,
+        'color': color,
+      };
+
+  factory SidecarText.fromJson(Map<String, dynamic> json) => SidecarText(
+        id: json['id'] as String,
+        nx: (json['nx'] as num).toDouble(),
+        ny: (json['ny'] as num).toDouble(),
+        text: (json['text'] as String?) ?? '',
+        fontSize: (json['fontSize'] as num?)?.toDouble() ?? 0.03,
+        color: (json['color'] as num?)?.toInt() ?? 0xFF000000,
+      );
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is SidecarText &&
+          runtimeType == other.runtimeType &&
+          id == other.id &&
+          nx == other.nx &&
+          ny == other.ny &&
+          text == other.text &&
+          fontSize == other.fontSize &&
+          color == other.color;
+
+  @override
+  int get hashCode => Object.hash(id, nx, ny, text, fontSize, color);
+
+  @override
+  String toString() =>
+      'SidecarText(id: $id, nx: $nx, ny: $ny, text: $text, '
+      'fontSize: $fontSize, color: $color)';
+}
+
 /// An anchor's private infinite scratchpad: a list of [InkStroke]s in ABSOLUTE
 /// world pixels (unchanged format from `SplitViewScreen`), plus the world size
 /// so it restores (today the canvas always resets to 4000×4000).
@@ -219,6 +308,7 @@ class BadnoteSidecar {
     this.updatedAt,
     Map<int, List<EditorStroke>>? strokes,
     Map<int, List<SidecarHighlight>>? highlights,
+    Map<int, List<SidecarText>>? texts,
     List<Bookmark>? bookmarks,
     List<SidecarScratchLink>? scratchLinks,
     Map<int, String>? legacyAnnotations,
@@ -227,6 +317,7 @@ class BadnoteSidecar {
     this.background,
   })  : strokes = strokes ?? <int, List<EditorStroke>>{},
         highlights = highlights ?? <int, List<SidecarHighlight>>{},
+        texts = texts ?? <int, List<SidecarText>>{},
         bookmarks = bookmarks ?? <Bookmark>[],
         scratchLinks = scratchLinks ?? <SidecarScratchLink>[],
         legacyAnnotations = legacyAnnotations ?? <int, String>{};
@@ -255,6 +346,10 @@ class BadnoteSidecar {
   /// Page index → highlighted text rects (normalized).
   final Map<int, List<SidecarHighlight>> highlights;
 
+  /// Page index → typed-text annotations (normalized position, page-relative
+  /// font size). PDF editor only for now (note text is a later increment).
+  final Map<int, List<SidecarText>> texts;
+
   final List<Bookmark> bookmarks;
   final List<SidecarScratchLink> scratchLinks;
 
@@ -304,6 +399,12 @@ class BadnoteSidecar {
             entry.key.toString():
                 entry.value.map((h) => h.toJson()).toList(),
         },
+        if (texts.isNotEmpty)
+          'texts': {
+            for (final entry in texts.entries)
+              entry.key.toString():
+                  entry.value.map((t) => t.toJson()).toList(),
+          },
         'bookmarks': bookmarks.map((b) => b.toJson()).toList(),
         'scratchLinks': scratchLinks.map((s) => s.toJson()).toList(),
         if (legacyAnnotations.isNotEmpty)
@@ -350,6 +451,7 @@ class BadnoteSidecar {
           : DateTime.tryParse(json['updatedAt'] as String),
       strokes: decodePageMap(json['strokes'], EditorStroke.fromJson),
       highlights: decodePageMap(json['highlights'], SidecarHighlight.fromJson),
+      texts: decodePageMap(json['texts'], SidecarText.fromJson),
       bookmarks: ((json['bookmarks'] as List<dynamic>?) ?? const [])
           .map((e) => Bookmark.fromJson(e as Map<String, dynamic>))
           .toList(),