fix(mobile): Ignore child-only history in Home empty state#13

What Changed

The mobile Home screen now shows its full-page "No threads yet" state when the only unarchived threads are delegated subagent children. Pending tasks and top-level forks still count as content, and a search or environment filter with no matches still shows the in-list no-results message.

Why

Home hides subagent children from its list, but the empty-state check counted them, so a user whose only history was child threads saw an empty list with no explanation.

UI Changes

Home shows the "No threads yet" state in that case instead of an empty list. No screenshots are attached.

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