fix(ui): a picture that fails to load shows the card's own frame, then retries

A bare <img> sent a Referer that Google's image CDNs refuse and had no retry and no fallback, so a
Home shelf card showed the browser's broken-image mark. ui/RemoteImage gathers what Avatar, the
channel banner and the download rows had each hand-rolled: no Referer, up to three retries past the
browser's negative cache, the caller's fallback (by default the card's empty frame) instead of the
broken mark, the failure remembered by URL. Every picture now goes through it; a lint rule refuses
a bare <img>.

frontend/src/lib/gates.snapshot.json

@@ -756,6 +756,10 @@
             "message": "No native <select>: use ui/SegmentedToggle for a short fixed set, ui/FilterSelect for a longer one or a user's own list.",
             "selector": "JSXOpeningElement[name.name='select']"
           },
+          {
+            "message": "No bare <img>: use ui/RemoteImage — no Referer, a retry past the browser's negative cache, and a fallback instead of the broken-image mark.",
+            "selector": "JSXOpeningElement[name.name='img']"
+          },
           {
             "message": "px-frozen font size doesn't scale with --font-scale: use text-2xs/text-3xs or text-[Nrem], not text-[Npx].",
             "selector": "Literal[value=/text-\\[[\\d.]+px\\]/]"