✨ feat(megadebrid): add API mode with toggle and provider labels

- Add Mega-Debrid API support (connectUser + getLink endpoints)
- API mode preferred by default, with automatic web fallback on failure
- User toggle "Mega-Debrid bevorzugt über API" in settings UI
- Provider labels now show source: "Mega-Debrid (API)" or "Mega-Debrid (Web)"
- sourceLabel propagated through all provider result paths
- API session token cached for 20 minutes with auto-invalidation
- Remove megaWebUnrestrict requirement for Mega-Debrid provider config

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/realdebrid.ts

@@ -9,6 +9,7 @@ export interface UnrestrictedLink {
   fileSize: number | null;
   retriesUsed: number;
   skipTlsVerify?: boolean;
+  sourceLabel?: string;
 }
 
 function shouldRetryStatus(status: number): boolean {