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/bold/check-circle.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><polyline points="88 136 112 160 168 104" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><circle cx="128" cy="128" r="96" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/></svg>

assets/icons/bold/info.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><circle cx="124" cy="84" r="16"/><circle cx="128" cy="128" r="96" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><path d="M120,124a8,8,0,0,1,8,8v36a8,8,0,0,0,8,8" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/></svg>

assets/icons/bold/question.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><circle cx="128" cy="180" r="16"/><circle cx="128" cy="128" r="96" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><path d="M128,140v-8c17.67,0,32-12.54,32-28s-14.33-28-32-28S96,88.54,96,104v4" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/></svg>

assets/icons/bold/spinner.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><line x1="128" y1="32" x2="128" y2="64" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="195.88" y1="60.12" x2="173.25" y2="82.75" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="224" y1="128" x2="192" y2="128" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="195.88" y1="195.88" x2="173.25" y2="173.25" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="128" y1="224" x2="128" y2="192" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="60.12" y1="195.88" x2="82.75" y2="173.25" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="32" y1="128" x2="64" y2="128" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="60.12" y1="60.12" x2="82.75" y2="82.75" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/></svg>

assets/icons/bold/warning-circle.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><circle cx="128" cy="128" r="96" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="128" y1="132" x2="128" y2="80" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><circle cx="128" cy="172" r="16"/></svg>

assets/icons/bold/warning.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><path d="M142.41,40.22l87.46,151.87C236,202.79,228.08,216,215.46,216H40.54C27.92,216,20,202.79,26.13,192.09L113.59,40.22C119.89,29.26,136.11,29.26,142.41,40.22Z" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="128" y1="136" x2="128" y2="104" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><circle cx="128" cy="176" r="16"/></svg>

assets/icons/bold/x-circle.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><line x1="160" y1="96" x2="96" y2="160" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><line x1="96" y1="96" x2="160" y2="160" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/><circle cx="128" cy="128" r="96" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="24"/></svg>