feat: remove PostHog usage telemetry entirely

Drop the once-a-day anonymous "app_opened" heartbeat to PostHog and everything
around it: telemetry.go (sendTelemetryHeartbeat, GetTelemetryEnabled/
SetTelemetryEnabled, install-ID/last-sent settings keys, PostHog host + API key)
and the startup goroutine that fired it. The 'Send anonymous usage statistics'
toggle is removed from Settings → General (TelemetryToggle + i18n keys), and the
telemetry mentions are stripped from the README and wiki.

appVersion (still used by changelog/update/livestatus/log-email) moves to a new
version.go; release.ps1 now rewrites it there instead of telemetry.go.

app.go

@@ -1120,9 +1120,6 @@ func (a *App) startup(ctx context.Context) {
 		}
 	}()
 
-	// Anonymous usage heartbeat (once/day) so we can gauge active users. No-op
-	// when disabled in Preferences or until the PostHog key is configured.
-	go a.sendTelemetryHeartbeat()
 	go a.liveStatusLoop() // multi-op: heartbeat current activity to shared MySQL
 	go a.chatLoop()       // multi-op: poll the shared chat + heartbeat presence