fix(downloads): harden recovery, replacement, and cleanup

- make media credential recovery durable and explicit across lifecycle entry points
- protect exact output replacement with platform-aware ownership, fingerprints, locks, and crash-safe quarantine
- permanently remove unfinished assets while preserving safe completed and Torrent cleanup
- add frontend, native, and localization regressions

src/utils/addDownloadMetadata.ts

@@ -54,6 +54,8 @@ export interface AddDownloadDraftRow {
   playlistError?: string;
   metadataBlockedReason?: 'unsafe-url';
   selected?: boolean;
+  /** Opaque native fingerprint captured for an exact unmanaged-file replace. */
+  replaceExistingFingerprint?: string;
   isTorrent?: boolean;
   torrentPath?: string;
   torrentCacheId?: string;