0.5.1: replace properties checkboxes with icon toggles

All checkboxes and radio buttons in the Properties panel are now
icon-toggle buttons:

- Polygon "Closed path" → PhPolygon button (fill weight when closed)
- Fill/Stroke transparent → PhEye / PhEyeSlash toggle next to the swatch
- Mask use → PhMaskHappy toggle
- Mask mode → segmented control with PhPlus / PhMinus (negative is red)

Shared .icon-toggle style: 26×26, panel background, accent when active,
optional .danger variant. Tests updated to click buttons instead of
checkboxes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "animatr",
   "private": true,
-  "version": "0.5.0",
+  "version": "0.5.1",
   "type": "module",
   "scripts": {
     "dev": "vite",