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.

wiki/Security.md

@@ -18,7 +18,5 @@ them.
 - **Callsign lookups** go to QRZ.com / HamQTH with your credentials.
 - **QSL uploads/downloads** go to the services you configure (LoTW via TQSL, QRZ,
   eQSL, ClubLog, HRDLog).
-- **Telemetry** is a once-a-day anonymous heartbeat (random install ID + version
-  + OS — no callsign, no QSO data). Opt out in Preferences.
 - Everything else stays local (or on your own MySQL / web server for the
   multi-op live status).