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.

frontend/wailsjs/go/main/App.js

@@ -898,6 +898,10 @@ 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);
 }
@@ -1706,6 +1710,10 @@ 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);
 }