Components.AI shells, conversation switching, and responsive layout

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

The page, drawer, and bubble shells duplicate behavior, have inconsistent slots and semantics, lack defined conversation-switch behavior, and do not adapt to mobile layouts.

### Describe the solution you'd like

Track conversation-switch semantics and implementation, shell accessibility and focus behavior, responsive layouts, slot parity, shared status/footer rendering, theming, and model/usage attribution hooks.

This is a workstream under #68227. Its independently mergeable tasks are represented as native sub-issues.

### Additional context

Covers A6, H3-H5, and L1-L6 from #68227.

Start with the native sub-issues for this workstream and read parent issue #68227 for the A6, H3-H5, and L1-L6 requirements. The independently mergeable sub-issues define the concrete entry points; done means completing one of those tasks while addressing its stated shell, conversation-switching, responsive-layout, or accessibility scope.

Written by the indexing model from the issue text.