perf(ui): migrate icons to SVG sprite system

Replace @remixicon/react with a shared Icon component that renders
via <use href> references to a single hidden SVG sprite. This reduces
DOM node count by replacing inline SVGs with lightweight references.

- Create Icon component with sprite injection (packages/ui/src/components/icon/)
- Migrate all 164 files from @remixicon/react to Icon component
- Auto-generate sprite data from remixicon bundle (scripts/generate-icon-sprite.mjs)
- Add bun run icons:generate to package.json
- Move @remixicon/react to devDependencies
- Add icon usage instructions to theme-system skill

packages/ui/package.json

@@ -42,7 +42,6 @@
     "@opencode-ai/sdk": "^1.4.41",
     "@pierre/diffs": "1.1.0-beta.13",
     "@base-ui/react": "^1.4.0",
-    "@remixicon/react": "^4.7.0",
     "@simplewebauthn/browser": "13.3.0",
     "@tanstack/react-virtual": "^3.13.18",
     "@types/react-syntax-highlighter": "^15.5.13",
@@ -88,6 +87,7 @@
     "@types/qrcode": "^1.5.5",
     "@types/react": "^19.1.10",
     "@types/react-dom": "^19.1.7",
+    "@remixicon/react": "^4.7.0",
     "@vitejs/plugin-react": "^5.0.0",
     "autoprefixer": "^10.4.21",
     "concurrently": "^9.2.1",