fix(mobile): Surface response fork failures#15

What Changed

Forking from an assistant response on mobile now tells the user when the fork request fails. An alert titled "Could not confirm fork" explains that the fork may or may not exist and asks them to check the thread list before retrying, to avoid a duplicate. A request cancelled because the screen went away stays silent. The fork button also gets a 44pt touch target with extra hit slop.

Why

A failed fork ended its spinner and did nothing, so users retried and sometimes created duplicate threads. The server may have created the fork even when the client saw an error, so the message says the outcome is unconfirmed rather than claiming it failed.

UI Changes

Adds the "Could not confirm fork" alert and enlarges the fork button's touch target. 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