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/screens/split_view_screen.dart

@@ -383,6 +383,9 @@ class _SplitViewState extends State<SplitViewScreen> {
                 active: _tool == CanvasTool.pen,
                 tooltip: 'Brush',
                 labelFor: brushLabelEn,
+                // The scratchpad keeps a single shared color (no per-brush color
+                // memory in this surface); show it for every brush.
+                colorFor: (_) => _color,
                 onSelected: (b) => setState(() {
                   _penBrush = b;
                   _tool = CanvasTool.pen;