Drop the external demo: the Widgets tab is the demo

The hamlab-demo site, its Widgets-tab button, the demo_url config key and the make publish
target that fed it are all gone, along with the public read-only token it needed. The
example stays in examples/external-demo as something to read. The installer one-liner
already comes from Gitea releases.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/Gallery.tsx

@@ -31,7 +31,7 @@ function Tile({ spec, data, note }: { spec: WidgetSpec; data: WidgetData; note:
   )
 }
 
-export default function Gallery({ demoUrl }: { demoUrl?: string }) {
+export default function Gallery() {
   const [cat, setCat] = useState<Sensor[] | null>(null)
   const [err, setErr] = useState('')
   const rate = useRefreshMs()
@@ -61,7 +61,6 @@ export default function Gallery({ demoUrl }: { demoUrl?: string }) {
         <span className="hint">{needle ? `${shown.length} of ${sensors.length}` : sensors.length} sensors on this machine · live · tap a tile to copy its ID</span>
         <input className="gallery-find" value={find} onChange={(e) => setFind(e.target.value)} placeholder="find — gitea, temp, enp3s0…" spellCheck={false} autoCapitalize="off" />
         <span className="spacer" />
-        {demoUrl && <a className="btn" href={demoUrl} target="_blank" rel="noreferrer" title="a page outside Ham Lab, built on the same sensor API">External demo ↗</a>}
       </div>
 
       {!needle && <h2 className="section">Widget types</h2>}