[Fix] Model list does not scroll inside New Session dialog

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

Make the searchable shared model picker own the active modal layer while open, so wheel and trackpad events reach its portaled list when the picker is nested inside the New Session dialog. Add focused regression coverage for the nested dialog path.

Why this change was made

The dialog scroll lock treated the non-modal model picker portal as outside the dialog content and canceled wheel events, leaving a visibly scrollable long model list stuck at its initial position.

Impact

Long model lists now scroll inside nested modal surfaces without changing search, keyboard selection, or the existing Radix Select path for eight or fewer models. Focused component tests and the repository pre-push checks pass.

Screencasts

The genuine local New Session surface shows the nested searchable list moving from the initial Claude/DeepSeek rows to later GLM/GPT/Grok rows while the overlay remains stable.

[roomote-community]

No code issues found. See task

_{Reviewed 350314c}