refactor: extract tile variants into Astro components

Move each tile variant (posts: cassette/circuit/blast, shots:
viewfinder/evidence/hud/hero, bsky: text/media, links: relay/dispatch,
lists) into individual components under src/components/tiles/.

Extract variant picker logic and helpers (excerpt, relTime,
extractListItems) into src/lib/variants.ts.

index.astro now imports components and delegates to them, reducing from
750+ lines of inline markup to a clean dispatch loop.