Rename basic catalog scaffolding examples for better clarity

As discussed in PR #1497 (https://github.com/a2ui-project/a2ui/pull/1497#discussion_r3416520470), the `00_` prefix on some basic catalog examples (e.g., `00_capitalized-text.json`) was used as a trick to sort them first.

To make their purpose more explicit and to make it easier to filter them out in public gallery applications (where they might not give a good impression of A2UI's capabilities if shown at the top), we should clearly label them as examples used to scaffold an app.

Suggested renaming:
- `00_capitalized-text.json` -> `scaffold_00_capitalized-text.json`
- `00_complex-layout.json` -> `scaffold_01_complex-layout.json`
- etc., or follow the numbering scheme used elsewhere so they are consistent.