feat(files): allow large manual downloads and offline files

[veryCrunchy]

User problem

Explicit user actions are blocked by fixed 64 MiB Download and 512 MiB available-offline limits. Camera and video files commonly exceed those limits. A person who deliberately requests a local copy should be limited by available storage and server policy, not by an arbitrary application cap.

Acceptance criteria

• Allow a person to download a remote file regardless of its declared size when local storage and server policy permit it.
• Allow a person to make an individual remote file available offline regardless of its declared size when local storage and server policy permit it.
• Use streaming writes to a temporary file followed by validation and atomic promotion; never hold the complete file in memory or expose a partial file as complete.
• Check destination capacity and the configured free-space reserve before transfer, including safe behavior for unknown file sizes.
• Show progress and support cancellation and resumable retry for large transfers.
• Preserve the selected ETag or generation and reject a mixed or changed remote version.
• Keep downloaded exports, offline pins, and disposable cache entries as distinct retention classes.
• Replace hard rejection with a useful storage warning when an operation may consume substantial space.
• Recover safely after process death, network loss, authentication expiry, quota failure, and a destination that becomes unavailable.
• Add deterministic coverage for multi-gigabyte metadata, unknown sizes, insufficient space, cancellation, resume, ETag changes, and atomic promotion failure.

Relationships

• Parent: [EPIC] Files client and advanced sync #11
• Builds on transfer progress and cancellation in [SYNC-006] Transfer progress and cancellation #112.
• Builds on resumable downloads in [SYNC-008] Resumable downloads #114.
• Coordinates durable offline work with [SYNC-010] Offline operation queue #116 and cache retention with [SYNC-021] Tiered virtual-file cache storage #321.