feat: add bluesky post tiles and feed to homepage grid

- Pinned bsky posts render as tiles in date order alongside fragments
- Text-only bsky tiles get inner frame + scanline treatment
- Image bsky tiles show the image with text overlay
- 2x4 feed tile shows recent bsky posts (excludes pinned ones)
- Log tile now includes bsky posts in the timeline
- New BskyPost type and fetch functions (getPostThread, getAuthorFeed)
- All fetching happens in parallel for fast builds