feat(icons): adopt Phosphor as the icon system, ship curated 80-icon set

- Restructure assets/icons: drop ui/, rename outline->regular, filled->fill, add bold/
- Pull 80 Phosphor icons in regular weight (all) + fill weight (all) + bold (status set)
- Include Phosphor MIT LICENSE
- Rewrite icons README with folder map, specs, usage examples, contribution rules

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

assets/icons/regular/eye-slash.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><line x1="48" y1="40" x2="208" y2="216" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M154.91,157.6a40,40,0,0,1-53.82-59.2" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M135.53,88.71a40,40,0,0,1,32.3,35.53" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M208.61,169.1C230.41,149.58,240,128,240,128S208,56,128,56a126,126,0,0,0-20.68,1.68" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M74,68.6C33.23,89.24,16,128,16,128s32,72,112,72a118.05,118.05,0,0,0,54-12.6" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/></svg>