docs: refresh mesh chat README screenshot for bubbles chat UI

Replace the transcript-style mesh conversation capture with the same
content in the current bubbles chat style: a received image, voice
notes from both sides, and text bubbles with author-colour washes.
Captured on an Android 17 emulator with the synthetic review fixture;
no real conversations or device data.

README.md

@@ -20,7 +20,7 @@ This is the Android implementation of bitchat, fully protocol-compatible with th
     <th>Geohash globe picker</th>
   </tr>
   <tr>
-    <td><img src="docs/screenshots/readme-mesh-chat.png" alt="Active four-peer Bitchat mesh conversation with an image, voice messages, and text messages" width="360"/></td>
+    <td><img src="docs/screenshots/readme-mesh-chat.png" alt="Bitchat mesh conversation in bubbles chat style: peer-coloured message bubbles with an image, voice messages, and text messages" width="360"/></td>
     <td><img src="docs/screenshots/readme-geohash-globe.png" alt="Bitchat geohash location picker showing the whole Earth and geohash grid" width="360"/></td>
   </tr>
 </table>