fix: top bar never wraps, and warn about 4K AV1 playback

The control row wrapped to a second line as the window narrowed. It is
now flex-nowrap with only the search field giving up width, and the
window minimum is raised to 1140x620 so the fixed controls always fit
beside the sidebar on one line.

Also lands the 4K AV1 warning in Settings. The earlier attempt at this
silently changed nothing — the replacement text did not match the file,
so the copy shipped unchanged while being reported as done.

src/components/Settings.tsx

@@ -148,9 +148,10 @@ export default function Settings({
             <section className="border-b border-slate-200 px-5 py-4 dark:border-slate-800">
               <SectionHeading>Download quality</SectionHeading>
               <p className={`mt-1.5 ${HELP}`}>
-                Above 1080p YouTube only serves VP9 and AV1. Those play here, but the
-                files are several times larger and older Macs may struggle. Pick 1080p
-                for H.264, which plays anywhere. Audio is AAC at every setting.
+                Above 1080p YouTube only serves VP9 and AV1, and <b>4K AV1 currently plays
+                back with visible artefacts in this player</b> — the downloaded file is
+                fine, it is the built-in decoder that struggles. 1080p gets H.264, which
+                plays cleanly and is several times smaller. Audio is AAC at every setting.
               </p>
               <label className="mt-2 grid grid-cols-[92px_1fr] items-center gap-2">
                 <span className={LABEL}>Quality</span>