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/editor/canvas/ink_painters.dart

@@ -199,6 +199,52 @@ class EraserPreviewPainter extends CustomPainter {
       old.pageSize != pageSize;
 }
 
+/// Paints the SELECT tool's selection: a dashed-ish bounding box around the
+/// selected stroke(s) so the user sees what is selected and draggable. The box
+/// is given in normalized page coords and scaled to pixels at paint time.
+class SelectionOverlayPainter extends CustomPainter {
+  SelectionOverlayPainter({
+    required this.boundsNorm,
+    required this.pageSize,
+  });
+
+  /// Selection bounding box in normalized page coords (null = nothing selected).
+  final Rect? boundsNorm;
+  final Size pageSize;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final b = boundsNorm;
+    if (b == null) return;
+    // Inflate slightly so the box doesn't clip the stroke's rendered width.
+    const padPx = 6.0;
+    final rect = Rect.fromLTRB(
+      b.left * pageSize.width - padPx,
+      b.top * pageSize.height - padPx,
+      b.right * pageSize.width + padPx,
+      b.bottom * pageSize.height + padPx,
+    );
+    canvas.drawRRect(
+      RRect.fromRectAndRadius(rect, const Radius.circular(4)),
+      Paint()
+        ..color = const Color(0xFF2962FF).withValues(alpha: 0.12)
+        ..style = PaintingStyle.fill,
+    );
+    canvas.drawRRect(
+      RRect.fromRectAndRadius(rect, const Radius.circular(4)),
+      Paint()
+        ..color = const Color(0xFF2962FF)
+        ..style = PaintingStyle.stroke
+        ..strokeWidth = 1.5
+        ..isAntiAlias = true,
+    );
+  }
+
+  @override
+  bool shouldRepaint(SelectionOverlayPainter old) =>
+      old.boundsNorm != boundsNorm || old.pageSize != pageSize;
+}
+
 /// Paints just the in-progress stroke (the live layer), kept behind its own
 /// RepaintBoundary so committed strokes don't repaint on every move.
 class LiveInkPainter extends CustomPainter {