Revert "feat: remove PostHog usage telemetry entirely"

This reverts commit 6a28344. Restore the once-a-day anonymous PostHog heartbeat
and its opt-out toggle: telemetry.go (sendTelemetryHeartbeat, GetTelemetryEnabled/
SetTelemetryEnabled, PostHog host + API key), the startup goroutine, the Settings
→ General toggle + i18n keys, and the README/wiki mentions. version.go is dropped
again (appVersion moves back into telemetry.go). release.ps1 (untracked) pointed
back at telemetry.go by hand.

app.go

@@ -1120,6 +1120,9 @@ 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