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_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: