feat(sync): add per-file mobile-data transfer policies

[veryCrunchy]

User problem

A sync pair currently chooses one network policy for every file: any connection or unmetered only. Users need smaller or important files to continue over mobile data while large files or selected media types wait for Wi-Fi or another unmetered network.

Acceptance criteria

• Allow ordered network-policy rules based on path, extension, MIME or media type, and file size.
• Support any validated connection, unmetered only, never transfer automatically, and an explicit one-time manual override.
• Keep deferred files pending with a clear waiting-for-unmetered-network state instead of marking them failed.
• Continue transferring eligible files from the same sync pair while restricted files wait.
• Make newly eligible work runnable after network transitions without duplicating operations.
• Preview which files are currently eligible and which will wait.
• Clearly state whether manual sync respects or temporarily overrides network policies.
• Use the same policy semantics for uploads and downloads unless directions are configured deliberately.
• Resume interrupted transfers without unnecessary restarts after a network transition.
• Add deterministic coverage for mixed-size queues, metered-to-unmetered transitions, roaming, loss of validation, policy edits, retries, and process death.

Relationships

• Parent: [EPIC] Files client and advanced sync #11
• Builds on pair-wide network constraints delivered through [SYNC-001] Functional network and power policies #107.
• Uses the metadata matcher tracked by feat(sync): add include and metadata-based sync rules #365.