fix(images): render grid + related covers as <img> (no jagged downscaling)
High-res line-art covers went jagged when shrunk into the small grid tiles / related cards
because they were CSS background-images (lower-quality GPU downscaling, worse on the
transform:scale(0.98) hover layer). Switched both to a real <img object-fit:cover>, which
browsers downsample with the high-quality image pipeline.
- views/partials/post-tile.ejs — grid cover = <img class=grid-tile-img>
- views/pages/post.ejs — related cover = <img class=post-related-img>
- assets/css/style.css — .grid-tile-img (absolute, object-fit:cover)
- views/shell.ejs — style.css cache buster v58