[mob][locker] Support saving file offline

[AmanRajSinghMourya]

Description

Users can select owned Locker files and choose Save offline

• Locker downloads the encrypted file immediately and stores it in app-private local storage
• a file is shown as offline only after that encrypted local copy exists
• users can later Unsave to remove the local offline copy

Testing

• save an owned file offline and verify the offline badge appears
• open an offline file without re-downloading the encrypted blob
• disable network and verify save-offline fails with immediate
  error and no offline mark
• unsave a file and verify the offline badge is removed
• trash an offline file and verify offline state is cleared
• restore a trashed file and verify it comes back as not offline

[chatgpt-codex-connector]

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