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

package.json

@@ -59,6 +59,7 @@
     "vscode:type-check": "bun run --cwd packages/vscode type-check",
     "docs:validate": "node scripts/docs/validate-docs.mjs",
     "icons:sprite": "node scripts/generate-file-type-sprite.mjs",
+    "icons:generate": "bun run scripts/generate-icon-sprite.mjs",
     "themes:port:opencode": "tsx scripts/port-opencode-theme.ts",
     "version:bump": "node scripts/bump-version.mjs",
     "release:prepare": "bun run build && bun run type-check && bun run lint",
@@ -105,7 +106,6 @@
     "@radix-ui/react-slot": "^1.2.3",
     "@radix-ui/react-toggle": "^1.1.10",
     "@radix-ui/react-tooltip": "^1.2.8",
-    "@remixicon/react": "^4.7.0",
     "@types/react-syntax-highlighter": "^15.5.13",
     "bun-pty": "^0.4.5",
     "class-variance-authority": "^0.7.1",
@@ -151,6 +151,7 @@
     "globals": "^16.3.0",
     "nodemon": "^3.1.7",
     "patch-package": "^8.0.0",
+    "@remixicon/react": "^4.7.0",
     "tailwindcss": "^4.0.0",
     "tsx": "^4.20.6",
     "tw-animate-css": "^1.3.8",