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_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';