fix(app): apply agent defaults to new-session drafts

[fobiat]

Issue for this PR

Fixes #38333.

Type of change

• Bug fix

What does this PR do?

Apply configured model/variant defaults to the new-session draft when selecting or cycling agents. Previously only the agent state changed, so a model copied from the previous tab still won at submission.

Agents without defaults retain the draft selection. Later manual choices remain usable. New-tab inheritance without an explicit agent-selection action is unchanged. Earlier related PRs #38372 and #40604 were closed without merging.

How did you verify your code works?

• Linux production browser test against a plugin-free 1.18.31 server and a local fake provider. Before: selecting Beta displayed Alpha/low and sent model=alpha, reasoning_effort=low. After: dropdown and keyboard cycling select Beta/high and send those values.
• Checked explicit variants, non-reasoning models, mid-generation selection, reconnect and session isolation using captured requests.
• App: 724 unit tests, 46 browser-condition tests, typecheck/build pass. Changed-file lint has zero warnings/errors; formatting passes. All 30 pre-push typecheck tasks pass.
• Ten production selection samples: median 30.05 ms before, 29.35 ms after. No timing threshold asserted.

Screenshots / recordings

Before:

After:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[fobiat]

Local gates and captured-provider browser verification are complete. The test, typecheck and nix-eval runs are action_required for fork-workflow approval. Could a maintainer approve those runs when reviewing?