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.

wiki/Settings-and-Data.md

@@ -47,3 +47,5 @@ format, or an **ADIF record on each logged QSO** — so external tools stay in s
 - **Autostart** external programs (WSJT-X, JTAlert, rotator control…) at launch,
   skipping any already running.
 - **Update check** at startup (toggleable).
+- **Anonymous usage telemetry** — a once-a-day heartbeat (random install ID +
+  version + OS; no callsign or QSO data). Opt out in Preferences.