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>

src/main/constants.ts

@@ -21,8 +21,8 @@ export const WRITE_BUFFER_SIZE = 512 * 1024;       // 512 KB write buffer (JDown
 export const WRITE_FLUSH_TIMEOUT_MS = 2000;         // 2s flush timeout
 export const ALLOCATION_UNIT_SIZE = 4096;           // 4 KB NTFS alignment
 export const STREAM_HIGH_WATER_MARK = 512 * 1024;       // 512 KB stream buffer — lower than before (2 MB) so backpressure triggers sooner when disk is slow
-export const DISK_BUSY_THRESHOLD_MS = 300;               // Internal detection threshold for disk backpressure
-export const DISK_BUSY_STATUS_THRESHOLD_MS = 500;        // Delay UI/log display for brief disk-write spikes
+export const DISK_BUSY_THRESHOLD_MS = 300;               // Internal detection threshold for disk backpressure
+export const DISK_BUSY_STATUS_THRESHOLD_MS = 500;        // Delay UI/log display for brief disk-write spikes
 
 export const SAMPLE_DIR_NAMES = new Set(["sample", "samples"]);
 export const SAMPLE_VIDEO_EXTENSIONS = new Set([".mkv", ".mp4", ".avi", ".mov", ".wmv", ".m4v", ".ts", ".m2ts", ".webm"]);
@@ -46,6 +46,7 @@ export function defaultSettings(): AppSettings {
     realDebridUseWebLogin: false,
     megaLogin: "",
     megaPassword: "",
+    megaCredentials: "",
     megaDebridApiEnabled: false,
     megaDebridWebEnabled: false,
     megaDebridPreferApi: true,
@@ -93,32 +94,36 @@ export function defaultSettings(): AppSettings {
     speedLimitMode: "global",
     updateRepo: DEFAULT_UPDATE_REPO,
     autoUpdateCheck: true,
-    clipboardWatch: false,
-    minimizeToTray: false,
-    theme: "dark" as const,
-    collapseNewPackages: true,
-    historyRetentionMode: "permanent",
-    accountListShowDetailedDebridLinkKeys: false,
-    autoSortPackagesByProgress: true,
-    autoSkipExtracted: false,
-    hideExtractedItems: true,
-    confirmDeleteSelection: true,
-    totalDownloadedAllTime: 0,
-    totalCompletedFilesAllTime: 0,
-    totalRuntimeAllTimeMs: 0,
-    bandwidthSchedules: [],
+    clipboardWatch: false,
+    minimizeToTray: false,
+    theme: "dark" as const,
+    collapseNewPackages: true,
+    historyRetentionMode: "permanent",
+    accountListShowDetailedDebridLinkKeys: false,
+    autoSortPackagesByProgress: true,
+    autoSkipExtracted: false,
+    hideExtractedItems: true,
+    confirmDeleteSelection: true,
+    totalDownloadedAllTime: 0,
+    totalCompletedFilesAllTime: 0,
+    totalRuntimeAllTimeMs: 0,
+    bandwidthSchedules: [],
     columnOrder: ["name", "size", "progress", "hoster", "account", "prio", "status", "speed"],
     extractCpuPriority: "high",
     autoExtractWhenStopped: true,
     disabledProviders: [],
-    hosterRouting: {},
-    providerDailyLimitBytes: {},
-    providerDailyUsageBytes: {},
-    providerTotalUsageBytes: {},
-    debridLinkApiKeyDailyLimitBytes: {},
-    debridLinkApiKeyDailyUsageBytes: {},
-    debridLinkApiKeyTotalUsageBytes: {},
-    providerDailyUsageDay: getProviderUsageDayKey(),
-    scheduledStartEpochMs: 0
-  };
-}
+    hosterRouting: {},
+    providerDailyLimitBytes: {},
+    providerDailyUsageBytes: {},
+    providerTotalUsageBytes: {},
+    debridLinkApiKeyDailyLimitBytes: {},
+    debridLinkApiKeyDailyUsageBytes: {},
+    debridLinkApiKeyTotalUsageBytes: {},
+    megaDebridDisabledAccountIds: [],
+    megaDebridAccountDailyLimitBytes: {},
+    megaDebridAccountDailyUsageBytes: {},
+    megaDebridAccountTotalUsageBytes: {},
+    providerDailyUsageDay: getProviderUsageDayKey(),
+    scheduledStartEpochMs: 0
+  };
+}