Add "Backup" action to download a project's /workspace as .tar.gz

Adds a manual backup button on each project card (next to Start/Reset
when stopped, and next to Files when running) that saves a gzipped
tarball of the container's /workspace to a host path via the native save
dialog.

Backend: download_container_backup runs `tar czf -` inside the container
(so excludes + compression happen there rather than streaming a 16 GB
workspace) and pipes stdout straight to the chosen file. Regenerable
build artifacts (node_modules, target, .git/objects) are excluded so the
archive stays restore-sized. Returns bytes written; stderr is captured
for error reporting and a zero-byte result is treated as failure.

Works whether the container is running or stopped (only requires that it
exists). Verified on the Ubuntu/GNU-tar container base.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src-tauri/src/lib.rs

@@ -184,6 +184,7 @@ pub fn run() {
             // Files
             commands::file_commands::list_container_files,
             commands::file_commands::download_container_file,
+            commands::file_commands::download_container_backup,
             commands::file_commands::upload_file_to_container,
             // MCP
             commands::mcp_commands::list_mcp_servers,