Add volume file browser and transfer commands

[ThallesP]

Summary

Adds volume file browsing and agent-friendly file transfer commands under railway volume.

Screen.Recording.2026-05-12.at.13.08.16.mov

• adds railway volume browse with a Ratatui remote browser, download/upload actions, and edit-and-sync for files
• adds non-interactive volume ls, volume download, and volume upload commands with JSON output support
• uses native SSH with a multiplexed control connection; file transfers use cat, directory transfers use streamed tar
• adds a local upload picker panel in the TUI instead of requiring typed upload paths

Why

Railway volume files need a simple interactive workflow for humans and simple commands for automation/agents. The SSH control connection also avoids repeated SSH key passphrase prompts during a session.

Validation

• cargo fmt --check
• cargo check

[ThallesP]

we are using ssh with tar + cat, totally not ideal but something is wrong with SCP impl: https://discord.com/channels/713503345364697088/1501995980227088474.

i can migrate to SCP if that bug gets fixed.