feat(website): add comprehensive accessibility improvements

Phase 1 - Screen Reader Critical:
- Add aria-label to main navigation
- Add accessible names to icon-only buttons (GitHub, theme toggle, close)
- Add aria-hidden to decorative SVGs and emoji icons
- Add role=dialog, aria-modal, aria-labelledby to modal
- Add skip link with visible focus state

Phase 2 - Keyboard Navigation:
- Implement focus trap in modal (Tab/Shift+Tab cycles)
- Return focus to trigger element on modal close
- Replace outline:none with visible focus rings
- Add keyboard navigation to install dropdown (arrows, escape)
- Add aria-expanded to dropdown toggles

Phase 3 - Dynamic Content:
- Add aria-live=polite to results counts and loading states
- Add role=listbox to search results
- Add role=list to resource lists
- Add role=menu/menuitem to dropdown menus

Phase 4 - Forms & Labels:
- Add .sr-only utility class for screen reader text
- Add visually hidden labels to all search inputs
- Add aria-label to filter dropdowns

Files modified:
- BaseLayout.astro, Modal.astro (ARIA attributes)
- modal.ts (focus trap, keyboard navigation)
- global.css (sr-only, skip-link, focus styles)
- All page files (labels, live regions, roles)

website/public/data/manifest.json

@@ -1,5 +1,5 @@
 {
-  "generated": "2026-02-01T23:32:16.397Z",
+  "generated": "2026-02-01T23:35:23.497Z",
   "counts": {
     "agents": 140,
     "prompts": 134,