docs(readme): swap the mobile idle screenshot for an interactive prompt

Replaces the middle cell of the Mobile-Optimized Web UI table in both
READMEs. The new shot shows an agent's multiple-choice prompt being answered
on a phone, with the touch accessory bar and bottom toolbar visible, which
demonstrates more of the mobile UI than the old idle-session capture.

Uses a dated filename per the convention the other 2026-07 images follow.
That also avoids GitHub's image cache serving the old picture, which an
in-place overwrite of mobile-session-idle.png would have risked. The old
file is left on disk so any existing external link to it keeps working.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -219,12 +219,12 @@ The most responsive AI coding agent experience on any phone. Full xterm.js termi
 <table>
 <tr>
 <td align="center" width="33%"><img src="docs/screenshots/mobile-landing-qr.png" alt="Mobile — landing page with QR auth" width="260"></td>
-<td align="center" width="33%"><img src="docs/screenshots/mobile-session-idle.png" alt="Mobile — idle session with keyboard accessory" width="260"></td>
+<td align="center" width="33%"><img src="docs/screenshots/mobile-session-question-20260727.png" alt="Mobile — answering an agent's multiple-choice prompt with the keyboard accessory bar" width="260"></td>
 <td align="center" width="33%"><img src="docs/screenshots/mobile-session-active.png" alt="Mobile — active agent session" width="260"></td>
 </tr>
 <tr>
 <td align="center"><em>Landing page with QR auth</em></td>
-<td align="center"><em>Keyboard accessory bar</em></td>
+<td align="center"><em>Answering prompts by touch</em></td>
 <td align="center"><em>Agent working in real-time</em></td>
 </tr>
 </table>