Add fullscreen difit integration

[mulyoved]

Summary

Add a fullscreen difit experience to T3 Code that can be toggled from the keyboard and viewed inside the existing app shell flow.

User-facing behavior

• Alt+G opens fullscreen diff from a thread
• the fullscreen view uses the active thread worktree or project root
• the diff viewer is loaded from a managed external difit process through a same-origin proxy path
• the fullscreen view closes with Alt+G, the floating back button, or Escape
• /restart-t3code is available as a project prompt to rebuild/restart the PM2-managed app

Implementation

• add a server-side difit manager for process startup, readiness, cwd resolution, and lifecycle
• add a reverse proxy layer for difit assets and API paths
• add a top-level fullscreen route in the web app
• add a dedicated difit.open RPC surface and shared contracts
• add keybinding handling for difit.toggle
• add browser/unit/server coverage for the new flow

Notes

This keeps the integration isolated in difit-specific files while limiting core changes to thin registration points in routing, keybindings, RPC wiring, and fullscreen entry.