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/api.ts

@@ -55,7 +55,7 @@ async function req<T>(method: string, path: string, body?: unknown, local = fals
 }
 
 export interface UpdateStatus { current: string; latest: string; arch: string; behind: boolean; checking: boolean; installing: boolean; enabled: boolean; auto: boolean; url: string; notes?: string; published?: string; checked?: string; error?: string }
-export interface Me { demo_url?: string; addr?: string; host: string; user: string; account?: string; update_available?: string; version: string; load: string; uptime: number; reboot_needed?: boolean; fingerprint?: string; arch?: string }
+export interface Me { addr?: string; host: string; user: string; account?: string; update_available?: string; version: string; load: string; uptime: number; reboot_needed?: boolean; fingerprint?: string; arch?: string }
 export interface PushResult { name: string; ok: boolean; version?: string; error?: string }
 // What a machine is called on screen: the hostname it reports, which follows a rename within
 // a poll. The name it was added under stays its identity — the URL, its accent colour, and