docs: refresh homepage screenshot and make it responsive

- Replace screenshot-main.png (both asset paths — the src/screenshot/
  copy referenced by index.md and the assets/images/ copy referenced
  elsewhere) with the current v1.1-beta.9 UI showing the widened
  Disconnect/Video buttons, the status dot replacing the label, the
  8px OSC dot, and the Users: N/M count.
- Wrap the homepage screenshot in <div class="screenshot"> instead of
  plain Markdown image syntax so it picks up the existing
  max-width:100% / rounded / shadowed responsive styling from
  style.scss (.screenshot rule).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mkschulze

docs/assets/images/screenshot-main.png

@@ -22,7 +22,9 @@ JamWide brings [NINJAM](https://www.cockos.com/ninjam/) — the open-source inte
 
 Load JamWide, connect to a server, and start jamming with musicians around the globe. Built with [JUCE](https://juce.com/) for native performance on macOS, Windows, and Linux.
 
-![JamWide Plugin Interface](assets/screenshot/screenshot-main.png)
+<div class="screenshot">
+  <img src="assets/screenshot/screenshot-main.png" alt="JamWide Plugin Interface" />
+</div>
 
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