fix(feed): serve full-resolution covers on mobile (responsive srcset)
The timeline card cover used a fixed 256px thumbnail — right for the 120px desktop
thumb, but the cover is full-width (100vw) on mobile, so a ~430px phone at 3x DPR
upscaled the 256px image ~5x → blurry (most visible in the PWA).
- src/views/partials/post-card.ejs — cover <img> now uses srcset (320/640/1280w) +
sizes "(min-width:768px) 120px, 100vw", so the browser picks ~1280 for the
full-width mobile cover and ~320 for the small desktop thumb (no over-fetch)
Co-Authored-By: Claude <noreply@…>