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_en.arb

@@ -91,6 +91,8 @@
   "actionHighlightSelection": "Highlight selection",
   "toolRemoveHighlight": "Remove highlight (tap a highlight)",
   "toolPlaceScratchLink": "Place scratch link",
+  "toolText": "Text (tap or double-click to add)",
+  "textPlaceholder": "Type…",
   "scratchLinkDeleteTitle": "Delete scratch link?",
   "scratchLinkDeleteBody": "This removes the anchor and its private scratchpad.",
   "toolAddBookmark": "Add bookmark (here or at selection)",

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:

lib/l10n/app_localizations_en.dart

@@ -247,6 +247,12 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get toolPlaceScratchLink => 'Place scratch link';
 
+  @override
+  String get toolText => 'Text (tap or double-click to add)';
+
+  @override
+  String get textPlaceholder => 'Type…';
+
   @override
   String get scratchLinkDeleteTitle => 'Delete scratch link?';
 

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？';
 

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）",