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/lib/tauri-commands.ts

@@ -76,6 +76,8 @@ export const listContainerFiles = (projectId: string, path: string) =>
   invoke<FileEntry[]>("list_container_files", { projectId, path });
 export const downloadContainerFile = (projectId: string, containerPath: string, hostPath: string) =>
   invoke<void>("download_container_file", { projectId, containerPath, hostPath });
+export const downloadContainerBackup = (projectId: string, hostPath: string, containerPath?: string) =>
+  invoke<number>("download_container_backup", { projectId, hostPath, containerPath });
 export const uploadFileToContainer = (projectId: string, hostPath: string, containerDir: string) =>
   invoke<void>("upload_file_to_container", { projectId, hostPath, containerDir });