Add layout tag search to /layouts browser

## Summary
Built-in layouts define `tags` in `slide.config.ts`, but the layouts browser only filters by category.

## Tasks
- [ ] Add tag filter chips or search input on `/layouts`
- [ ] Include tags in API layout list response if not already exposed
- [ ] Sort/filter by tag match

## Files
- `apps/web/app/layouts/LayoutsBrowser.tsx`
- `apps/api/src/routes/layouts.ts`
- `layouts/title-hero/slide.config.ts` (example tags)

Start with apps/web/app/layouts/LayoutsBrowser.tsx to trace the existing category filtering, then inspect apps/api/src/routes/layouts.ts to confirm whether layout tags are already returned. Use layouts/title-hero/slide.config.ts as the tags example. Done means /layouts exposes a tag search or filter, matches layouts by tags, and preserves the existing category browsing.

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