Add docker cp equivalent in Files panel

[GustavoCaso]

Summary

Enable copying files between a container and the host filesystem from within the Files panel, equivalent to docker cp.

Current State

• The Files panel (internal/ui/sections/containers/panel/filetree_panel.go) shows the container filesystem as a browsable tree
• Files can be browsed but not copied to/from the host

Implementation Steps

1. Add client interface methods

In internal/client/client.go, add to ContainerService:

CopyFromContainer(ctx context.Context, containerID, srcPath string) (io.ReadCloser, error)
CopyToContainer(ctx context.Context, containerID, dstPath string, content io.Reader) error

2. Implement in Docker client

Use client.CopyFromContainer and client.CopyToContainer. The Docker API returns/accepts a tar stream.

3. Copy from container to host

• Add key binding (e.g. c) in the Files panel when a file is highlighted
• Prompt user for destination path on the host (default: current working directory)
• Extract the tar stream to the destination
• Show success/error banner with the file path written

4. Copy from host to container

• Add key binding (e.g. u for upload) in the Files panel
• Prompt user for the source file path on the host
• Prompt for destination directory in the container (default: current directory in tree)
• Tar the file and send via CopyToContainer
• Refresh the file tree after upload

5. Progress indicator

Show a spinner with filename during copy operations.

Acceptance Criteria

• Pressing c on a selected file copies it to a host directory
• Pressing u uploads a host file to the current container directory
• Destination path is user-configurable with a sensible default
• Large files show a progress/spinner indicator
• File tree refreshes after upload
• Error shown if path does not exist or permissions are denied