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.

frontend/wailsjs/go/main/App.js

@@ -898,10 +898,6 @@ export function GetStationStatus() {
   return window['go']['main']['App']['GetStationStatus']();
 }
 
-export function GetTelemetryEnabled() {
-  return window['go']['main']['App']['GetTelemetryEnabled']();
-}
-
 export function GetUIPref(arg1) {
   return window['go']['main']['App']['GetUIPref'](arg1);
 }
@@ -1710,10 +1706,6 @@ export function SetPassphrase(arg1) {
   return window['go']['main']['App']['SetPassphrase'](arg1);
 }
 
-export function SetTelemetryEnabled(arg1) {
-  return window['go']['main']['App']['SetTelemetryEnabled'](arg1);
-}
-
 export function SetUIPref(arg1, arg2) {
   return window['go']['main']['App']['SetUIPref'](arg1, arg2);
 }