Toolbar icon redesign: pen dot, line/box icons, undo/redo arrows

- Pen sizes consolidated into single button with dot icon showing
  current size; long-press for 0.38/0.5/0.6/0.7mm selection
- Line button shows thick line icon; long-press for style variants
- Box button shows rectangle outline icon
- Undo/redo replaced with curved arrow icons (gray when disabled)
- Added PenSize enum (MM_038, MM_050, MM_060, MM_070)
- Removed PEN_FINE/PEN_MEDIUM tools, replaced with single PEN tool
  that uses the selected PenSize from CanvasState

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -82,7 +82,8 @@ eng-pad/
 - Stroke points are packed as little-endian float BLOBs: `[x0,y0,x1,y1,...]`
 - All coordinates are in canonical space (300 DPI). Screen transform via `Matrix`.
 - Grid drawn in **screen space** with pixel-snapped positions (not canonical space) for uniform squares.
-- Pen widths: 0.38mm = 4.49pt, 0.5mm = 5.91pt (at 300 DPI).
+- Pen widths: 0.38mm (4.49pt), 0.5mm (5.91pt), 0.6mm (7.09pt), 0.7mm (8.27pt) at 300 DPI.
+- Single PEN tool with long-press for size selection; remembers last size.
 - Anti-aliasing disabled on all paint objects for crisp e-ink rendering.
 - No backing bitmap — strokes draw directly as paths.
 - Viewport: page always fills screen, pan clamped to edges, dynamic min zoom.