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.d.ts

@@ -472,8 +472,6 @@ 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>;
@@ -878,8 +876,6 @@ 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>;