FUSE: Implement Unmount feature

[efimsky]

Parent Issue

Part of #118 (FUSE-based Archive & Remote Browsing Feature)

Summary

Add ability to unmount the current mounted filesystem from the menu.

Dependencies

Requires #123 (rclone integration) - menu structure already in place

Tasks

Action Enum

• Add UNMOUNT = auto() to Action enum

Menu Integration

• Add 'unmount': Action.UNMOUNT to MENU_ACTION_MAP
• "Unmount current" menu item should already exist from Phase 5

Dynamic Menu State

• Disable "Unmount current" when active panel is NOT inside a mount
• Enable when active panel path is inside an active mount

App Method

• Handle Action.UNMOUNT in run loop
• Implement App._unmount_current():
  • Get current path from active panel
  • Find mount using mount_manager.get_mount_for_path()
  • If not in mount, show error
  • Navigate panel to parent directory (outside mount)
  • Call mount_manager.unmount(mount_point)
  • Show success/failure message

Edge Cases

• Handle unmounting when panel is deep inside mount (navigate to mount's parent)
• Handle unmount failure gracefully (show error, don't crash)

Testing

• Test UNMOUNT action triggers unmount
• Test unmount navigates panel outside mount before unmounting
• Test unmount fails gracefully when not in mount
• Test "Unmount current" menu item enabled/disabled based on path

Files

• Modify: tnc/app.py
• Modify: tnc/menu.py