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/Shell.tsx

@@ -269,7 +269,7 @@ export default function Shell({ me: hubMe, onLoggedOut }: { me: Me; onLoggedOut:
           {view === 'space' && <Space key={machine ?? ''} />}
           {view === 'machines' && <Machines hub={hubMe} onChanged={loadMachines} onOpen={(n) => { switchMachine(n); setView('dashboard') }} />}
           {view === 'dashboard' && <Dashboard />}
-          {view === 'widgets' && <Gallery demoUrl={hubMe.demo_url} />}
+          {view === 'widgets' && <Gallery />}
           {view === 'docker' && <Docker />}
           {view === 'alerts' && <Alerts machines={machines.map((m) => m.name)} titleOf={(n) => titleOf(n === hubMe.host ? null : n, machines, hubMe.host)} />}
           {view === 'top' && <Top />}