Add command palette (Cmd+K) with filtered search

Pagefind already indexes everything (integrations/pagefind.mjs) but the current SearchDropdown is pretty basic.

I'd like a proper command palette:
- open with Cmd+K / Ctrl+K and `/`, close with Esc, trap focus properly
- search posts, projects, tags, and pages in one place with grouping
- filters: by tag, by year, by reading time (re-use logic from src/lib/tags.ts and src/lib/calculateReadingTime.ts)
- keyboard navigation with arrow keys, Enter to go, and recent searches stored in localStorage
- replace SearchDropdown.tsx in Header.astro for both desktop and mobile nav
- log search analytics so I know what people can't find

Look at how Linear and Algolia do it. Keep it fast, no extra JS until the palette opens.