Components.AI: Build a full built-in UI sample

### Is your feature request related to a problem? Please describe the problem.

There is no single sample demonstrating how the independently extensible pieces compose into the target experience.

### Describe the solution you'd like

Build a representative sample using the shipped composer parts, suggestions, attachments, actions, decorators, media, voice, tool-call modes, approvals, markdown wiring, switching, and responsive shells. Keep sample-specific design separate from core defaults.

### Additional context

Leaf task for #68227 requirement M4.

Start with the parent issue #68227 and its requirement M4, then inspect the shipped composer parts named here: suggestions, attachments, actions, decorators, media, voice, tool-call modes, approvals, markdown, switching, and responsive shells. Done means a representative built-in UI sample composes these pieces while keeping sample-specific design separate from core defaults.

Written by the indexing model from the issue text.