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.
