v1.15.0 — upload/download files & folders + cross-box transfer

The Files manager can now move files to and from your computer, and between boxes.

From your computer (Files modal)

• 📤 Upload and 📤 Folder — send one or many files, or a whole folder (its structure rebuilds on the box).
• Per-row ⬇ download — a file streams to your browser; a folder comes back as a .zip.
• Everything stays jailed to the manager's allowed roots; 2 GB/file cap. Works for the controller and any remote box through the SSH tunnel.

Between boxes (SCP over SSH)

• Per-row ↗ "send to another box" (shown when you have boxes registered). Pick a destination box and folder; the controller copies it with scp over its stored SSH credentials.
• Handles controller↔box and box→box (relayed through the controller). Each side is jailed to that box's roots.

No new dependencies — pure stdlib + the system scp/ssh already used for tunnels.