Add multi-select filters, light/dark theme, and skill ZIP downloads

- Add multi-select dropdown component for all filter fields
- Implement light/dark theme toggle with system preference detection
- Add client-side ZIP download for skills using JSZip
- Include file lists in skills metadata for download feature
- Add title tooltips to multi-select options for long values
- Update all pages with consistent theme toggle in header

website/data/manifest.json

@@ -1,5 +1,5 @@
 {
-  "generated": "2026-01-28T02:42:05.621Z",
+  "generated": "2026-01-28T03:53:29.513Z",
   "counts": {
     "agents": 140,
     "prompts": 134,