Add Mega-Debrid multi-account support with automatic fallback

Multiple Mega-Debrid accounts can now be configured as login:password
pairs (one per line). When an account hits Fair-Use limits or errors,
the next account is tried automatically.

- New parser module mega-debrid-accounts.ts (parse, ID generation,
  masking, serialization)
- Per-account daily limits, usage tracking, enable/disable
- Account rotation with per-mode cooldowns (API failures don't
  block Web attempts)
- Backward compatible: existing single megaLogin/megaPassword
  is auto-migrated to the new format
- UI: textarea for credentials, account list with masked logins

Follows the existing Debrid-Link multi-key pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/download-manager.test.ts

@@ -6167,6 +6167,7 @@ describe("download manager", () => {
         ...defaultSettings(),
         megaLogin: "mega-user",
         megaPassword: "mega-pass",
+        megaCredentials: "mega-user:mega-pass",
         megaDebridWebEnabled: true,
         megaDebridApiEnabled: false,
         megaDebridPreferApi: false,
@@ -9391,6 +9392,7 @@ describe("download manager", () => {
         ...defaultSettings(),
         megaLogin: "mega-user",
         megaPassword: "mega-pass",
+        megaCredentials: "mega-user:mega-pass",
         megaDebridApiEnabled: true,
         providerDailyUsageDay: getProviderUsageDayKey(),
         providerDailyUsageBytes: { realdebrid: 512 },