url-source: Collections with folders

## Summary
Organize url-source saved requests ("Collections") into folders.

## Scope (agreed: one level, explicit field)
- Add an optional `folder` field to `SavedRequest`/`State` (`plugins/url-source/src/lib.rs`), `#[serde(default)]`.
- On Save, add a "Folder" text input next to the request name (empty = ungrouped).
- Sidebar groups saved requests by folder under **collapsible** folder headers (DSL `collapsible` node); ungrouped requests listed below.
- List-item indices must map back to the flat `saved_requests` vec so the existing click/delete handler keeps working (emit the global index).

## Out of scope (future)
- Nested folders, drag-and-drop, rename/move UI.

## Notes
Split out of the post-#80 request-UI polish batch; the host `collapsible` DSL node already exists, so this is plugin-side.