Model page: Diagrams and Registry as collapsible panels, one header per folder

- <details> panels for Diagrams (open) and Registry (closed by default)
- each folder inside Diagrams is its own collapsible header with a count,
  above its diagram cards; diagrams next to main.archify list directly
- open/closed state is kept in the webview state, so re-renders and
  restores keep it

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - **Models:** a folder with a `main.archify` at its top is a model, a set of diagrams that belong together. Opening `main.archify` shows the model page: name and description, the overview diagram if the file has one, the diagrams grouped by folder (click to open), and the registry with where each element is used.
 - **Registry:** `*.archify-registry` files define shared elements once; diagrams draw them with `ref: <id>` plus position, and may override `label`, `sublabel` and `tag`. Hover, go-to-definition and completion on `ref`; unknown names, forbidden overrides, duplicate ids and broken registries in Problems. Registry edits re-render open diagrams.
+- **Model page panels:** Diagrams and Registry are collapsible panels; inside Diagrams every folder has its own collapsible header above its diagram cards. Open or closed is remembered.
 - **Cards:** a model diagram without `cards` of its own gets Archify cards from the registry descriptions of the elements it draws; the model page shows the registry as the same cards, each with the diagrams it is used in.
 - **Links:** `diagram:` on a registry element or a component points at the diagram that explains it; double-click the node (2D view) to open it.
 - **New Model…:** right-click a folder and choose *Archify | New Model…* (also in *File → New File…* and the Command Palette) to start from one of three complete C4 examples: **C4 - Standard Web App**, **C4 - E-Shop**, **C4 - Event-Based Integration**. Each creates a model folder with `main.archify`, a registry and context, container and component diagrams, named after the folder you choose, and opens the model page.