feat(tools): rnote-style toolbar core writing batch

Replace the ad-hoc tool palette with a shared tool system
(EditorToolKind) across the PDF, note and slide editors, and add
the core writing tools.

- Multiple brushes, each remembering its OWN color (rnote-style):
  selecting a brush restores its color, changing color updates only
  that brush, and each brush button shows its current color.
- Select tool: tap-select a committed stroke, drag to move it,
  delete it — persisted and undoable.
- Shape tool: line / rectangle / ellipse / arrow, drawn with a live
  preview and committed as generated PenStrokes (shape_geometry.dart)
  so they reuse stroke rendering, erase, persistence and undo.
- Highlighter + eraser fold into the same tool system.

Text/bookmark/search+OCR/backgrounds/Windows-Ink are later batches
(TODO). Brush opacity still deferred. analyze clean, 302 tests.

lib/l10n/app_en.arb

@@ -51,6 +51,14 @@
   "brushBallpoint": "Ballpoint",
   "brushPencil": "Pencil",
   "brushHighlighter": "Highlighter",
+  "toolSelect": "Select",
+  "toolShape": "Shape",
+  "shapePicker": "Shape",
+  "shapeLine": "Line",
+  "shapeRectangle": "Rectangle",
+  "shapeEllipse": "Ellipse",
+  "shapeArrow": "Arrow",
+  "actionDeleteSelection": "Delete selection",
   "actionUndo": "Undo",
   "actionRedo": "Redo",
   "fingerDrawingOn": "Finger drawing ON",

lib/l10n/app_localizations.dart

@@ -350,6 +350,54 @@ abstract class AppLocalizations {
   /// **'Highlighter'**
   String get brushHighlighter;
 
+  /// No description provided for @toolSelect.
+  ///
+  /// In en, this message translates to:
+  /// **'Select'**
+  String get toolSelect;
+
+  /// No description provided for @toolShape.
+  ///
+  /// In en, this message translates to:
+  /// **'Shape'**
+  String get toolShape;
+
+  /// No description provided for @shapePicker.
+  ///
+  /// In en, this message translates to:
+  /// **'Shape'**
+  String get shapePicker;
+
+  /// No description provided for @shapeLine.
+  ///
+  /// In en, this message translates to:
+  /// **'Line'**
+  String get shapeLine;
+
+  /// No description provided for @shapeRectangle.
+  ///
+  /// In en, this message translates to:
+  /// **'Rectangle'**
+  String get shapeRectangle;
+
+  /// No description provided for @shapeEllipse.
+  ///
+  /// In en, this message translates to:
+  /// **'Ellipse'**
+  String get shapeEllipse;
+
+  /// No description provided for @shapeArrow.
+  ///
+  /// In en, this message translates to:
+  /// **'Arrow'**
+  String get shapeArrow;
+
+  /// No description provided for @actionDeleteSelection.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete selection'**
+  String get actionDeleteSelection;
+
   /// No description provided for @actionUndo.
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_en.dart

@@ -140,6 +140,30 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get brushHighlighter => 'Highlighter';
 
+  @override
+  String get toolSelect => 'Select';
+
+  @override
+  String get toolShape => 'Shape';
+
+  @override
+  String get shapePicker => 'Shape';
+
+  @override
+  String get shapeLine => 'Line';
+
+  @override
+  String get shapeRectangle => 'Rectangle';
+
+  @override
+  String get shapeEllipse => 'Ellipse';
+
+  @override
+  String get shapeArrow => 'Arrow';
+
+  @override
+  String get actionDeleteSelection => 'Delete selection';
+
   @override
   String get actionUndo => 'Undo';
 

lib/l10n/app_localizations_zh.dart

@@ -140,6 +140,30 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get brushHighlighter => '荧光笔';
 
+  @override
+  String get toolSelect => '选择';
+
+  @override
+  String get toolShape => '形状';
+
+  @override
+  String get shapePicker => '形状';
+
+  @override
+  String get shapeLine => '直线';
+
+  @override
+  String get shapeRectangle => '矩形';
+
+  @override
+  String get shapeEllipse => '椭圆';
+
+  @override
+  String get shapeArrow => '箭头';
+
+  @override
+  String get actionDeleteSelection => '删除所选';
+
   @override
   String get actionUndo => '撤销';
 

lib/l10n/app_zh.arb

@@ -42,6 +42,14 @@
   "brushBallpoint": "圆珠笔",
   "brushPencil": "铅笔",
   "brushHighlighter": "荧光笔",
+  "toolSelect": "选择",
+  "toolShape": "形状",
+  "shapePicker": "形状",
+  "shapeLine": "直线",
+  "shapeRectangle": "矩形",
+  "shapeEllipse": "椭圆",
+  "shapeArrow": "箭头",
+  "actionDeleteSelection": "删除所选",
   "actionUndo": "撤销",
   "actionRedo": "重做",
   "fingerDrawingOn": "手指书写：开",