locker: cache encrypted files and harden download/open paths

[neeraj-pilot]

Summary

• switch file-open caching to encrypted blobs keyed by uploadedFileID
• persist encrypted cache only after successful decryption
• add decrypted cache files keyed by uploadedFileID and reuse them on open
• avoid decrypted temp file collisions by including uploadedFileID in non-cache output filename
• make resumable download manager recover when a stale completed temp file is missing
• clear locker cache directory contents as the last step during logout

Why

• stop relying on filename-based cache keys
• reduce redundant decrypt work on repeated opens
• fix failure to re-open large files after logout/login in the same app session
• prevent same-name file conflicts during repeated/overlapping open or download flows

Behavioral changes

• open flow now checks ID-keyed decrypted cache first
• if decrypted cache is missing, open flow reuses cached encrypted data (cache/<uploadedFileID>.encrypted) and decrypts
• decrypted cache path is ID-keyed (cache/<uploadedFileID>.decrypted<ext>), not displayName-keyed
• logout removes both encrypted and decrypted cache files

Validation

• flutter analyze lib/services/configuration.dart lib/services/files/download/file_downloader.dart lib/services/files/download/manager.dart lib/utils/file_util.dart
• analyzer reports no issues