fix(desktop): default submodule checkout timeout to 30 seconds

[bobzhang]

Workspaces without a saved timeout now allow 30 seconds instead of 10 for submodule checkout when creating or repairing worktrees. Update both the host default and frontend fallback, and assert the default explicitly in the existing settings test. Saved workspace overrides continue to apply.

Validation: five focused workspace-settings tests passed, along with just build, moon info, moon fmt, and git diff --check. No public interface changes. just check and just test are blocked by existing tuple-loop syntax errors in editor/viewer/diff_provider/moondiff/provider_wbtest.mbt and semantic_review_plan_test.mbt.