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.

README.md

@@ -276,6 +276,8 @@ only writes to the DB — it is not a web server.
   with a toast (toggleable), plus a **What's new** dialog that shows the changelog
   (English / French) on the first launch after an update — reopenable any time
   from the Help menu.
+- **Anonymous usage telemetry** (a once-a-day heartbeat: random install ID +
+  version + OS — no callsign or QSO data; opt-out in Preferences).
 
 ---