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.d.ts

@@ -472,6 +472,8 @@ export function GetStationSettings():Promise<main.StationSettings>;
 
 export function GetStationStatus():Promise<Array<main.StationDeviceStatus>>;
 
+export function GetTelemetryEnabled():Promise<boolean>;
+
 export function GetUIPref(arg1:string):Promise<string>;
 
 export function GetUltrabeamSettings():Promise<main.UltrabeamSettings>;
@@ -876,6 +878,8 @@ export function SetDVKLabel(arg1:number,arg2:string):Promise<void>;
 
 export function SetPassphrase(arg1:string):Promise<void>;
 
+export function SetTelemetryEnabled(arg1:boolean):Promise<void>;
+
 export function SetUIPref(arg1:string,arg2:string):Promise<void>;
 
 export function SetUltrabeamDirection(arg1:number):Promise<void>;