Add lightweight SFTP file management

[askac]

Why

SSH tabs provide interactive terminal access but no bounded way to move and manage reference files. The main-page context menu is also unavailable while a terminal is in Document Picture-in-Picture. Separately, Windows warm launches repeat a disposable PiP capability probe, and session recovery can reconnect before the previous Socket.IO transport finishes closing.

What changed

• Add a lightweight direct-endpoint SFTP File Manager with flat directory navigation, drag-and-drop upload, Keep Both or atomic Replace, download, rename, and two-stage permanent deletion.
• Bind regular-file actions to short-lived opaque references and browser-, socket-, terminal-, and live-bridge-scoped transfer tickets; nested interactive SSH sessions, symbolic links, and non-regular files remain outside the supported boundary.
• Expose SFTP through the main status bar, context menu, and Terminal PiP, while preserving hidden controls for non-SSH tabs and showing a disabled unavailable state after a typed backend failure.
• Skip the disposable PiP probe on Windows warm starts and defer session-recovery reconnect to the next event-loop task so the prior transport closes cleanly.

Testing

• Covered the complete backend suite, including SFTP endpoint scope, atomic replacement, opaque references, ticket binding, single-use downloads, and exact byte streaming.
• Covered the complete Chromium suite, including main and PiP SFTP entry points, unavailable states, upload conflicts, download dispatch, rename/delete safeguards, and recovered socket sessions.
• Verified a real HTTPS Edge download from an SSH tab through ticket acceptance to exact completion at 29,996,934 bytes; existing terminal, agent, SSH settings, rendering, and browser-authorization scenarios remain covered.