feat(artifacts): add a folder to the workspace (recursive import)

[juacker]

What

Add a folder (recursively) to the workspace artifacts. Previously the "+" import
dialog only accepted regular files — picking a folder failed with
"… is not a regular file".

How

• Backend — workspace_import_files now accepts directory sources and
  copies them recursively, reusing the hardened copy from feat(artifacts): drag-and-drop copy of artifacts between workspaces #109
  (copy_artifact_to_unique_destination → copy_dir_recursive):

  • symlinks and protected/heavy dirs (.git, node_modules, target, …) are
    skipped, so a folder import doesn't drag in VCS/build state;
  • destination names are collision-safe ( (n) suffix);
  • regular files still take the exact same path as before.
    copy_artifact_to_unique_destination is promoted to pub(crate) to share it.

• Frontend — a dedicated "Add folder" icon in the artifacts drawer
  header, next to "Add files". It's a separate action because native OS file
  dialogs can't select files and directories in one pass
  (openFileDialog({ directory: true, multiple: true })).

Verification

• cargo fmt / clippy -D warnings clean.
• Recursive copy is covered by the existing
  copy_artifact_handles_files_dirs_collisions_and_skips unit test.
• tsc / eslint / vitest (162) / vite build clean.

Test locally

Artifacts drawer → the new folder icon → pick a directory → it lands under the
workspace root (recursively, .git/target/etc. excluded), with a (n)
suffix on name collisions. "Add files" is unchanged.