fix(ui): respect display density and default icon sizes - #1728

Icon selection currently compares raster dimensions with a 40 CSS-pixel tile, which selects undersized images on high-density displays. Use the current device pixel ratio when scoring candidates: for example, a 2× display now chooses a 96×96 icon over a 48×48 icon for that tile.

Treat omitted or empty sizes like "any", matching the schema's default that an icon without sizes is usable at any size. This keeps an SVG without size metadata from losing to a fixed-size raster icon.

Follow-up to #1586.

Validation:

• Focused JavaScript assertions against the actual HTML helpers passed for default/1×/1.5×/2×/3× density, omitted/empty/any sizes in both candidate orders, undersized fallback, theme preference, unsafe schemes, and attribute escaping.
• go test ./internal/api/handlers/v0 -run '^TestUIHTML_' -count=1 passed.
• git diff --check passed.