[misantronic]

Ports the Linux watch-folder feature (#229) to Android.

• Folders added through the scan button are saved in the cache table (no schema change, so no DB version bump) and listed from the cached games header (x cached · y folders).
• The folders dialog toggles watching per folder (eye icon) and can remove a folder.
• ProxyService rescans watched folders once per budget window, hashes and queues only unseen ROMs, and the cache queue caches them within the budget.
• Stands down while a game is being played, offline, or during a manual bulk run.

Verified on an Odin2 Portal: a ROM copied into a watched folder was queued and cached on the next scan.