fix(clients): Prefer last-used models over project defaults#8

What Changed

New threads now start on the model you used last instead of the project's configured default model. On the web, a new thread first carries the model from the thread you are viewing, then falls back to the last app-wide selection, and uses the project default only when neither exists. Switching the project of an unsent draft follows the same order. On mobile, an unsent draft keeps its explicit pick, then the last app-wide selection wins over the project default and the provider default.

Explicit picks in a draft still stand, and the project's other settings, such as environment mode, worktree and branch defaults, resolve as before.

Why

Project defaults kept overriding a model the user had just chosen, so every new thread needed the model changed again. The project default is now a fallback for users who have not picked a model yet.

UI Changes

The model shown in the composer for a new thread changes as described. No screenshots are attached.

Known Limitations

On the web, the carried or last-used model is applied without first checking that its provider is available in the destination environment. Mobile already filters by availability. A follow-up should fall back to the next source when the provider is missing there.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes