#skip-bb

fixes https://github.com/dyad-sh/dyad/issues/2318

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## Summary by cubic
Persist the selected device mode (desktop, tablet, mobile) in user
settings so it stays after app rebuilds. Adds an end-to-end test to
verify the mode persists through a rebuild.

- **New Features**
- PreviewIframe reads and writes previewDeviceMode via useSettings,
replacing local state.
  - Added DeviceModeSchema and previewDeviceMode to UserSettingsSchema.

- **Bug Fixes**
- Stabilized e2e: added a persistence test, use po.clickRebuild(), and
wait for preview loading to appear/disappear with a final assertion
timeout.

<sup>Written for commit c801c37c314413ba23c0174dc7bb401193826389.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>