refactor(desktop): LemonSqueezy API 코드 완전 제거 (~150줄)

- LicenseService: LS API 상수/인터페이스/메서드 4개 삭제
- activate(): 로컬 키 검증 전용으로 단순화
- deactivate(): LS API 호출 제거
- initialize(): _tryPeriodicValidation 제거
- site/ taxNote: LemonSqueezy → Payple (10 locale)

site/src/i18n/locales/de.ts

@@ -97,7 +97,7 @@ export const de: Translations = {
     downloadFree: 'Gratis herunterladen',
     getPro: 'Pro holen',
     getProPlus: 'Pro+ holen',
-    taxNote: 'Alle Preise zzgl. MwSt. Sichere Zahlung uber LemonSqueezy.',
+    taxNote: 'Alle Preise zzgl. MwSt. Sichere Zahlung uber Payple.',
     rows: [
       { feature: 'Sprachdiktat', free: '15/Tag', pro: true, proPlus: true },
       { feature: 'KI-Textkorrektur', free: '3/Tag', pro: true, proPlus: true },