fix: prevent live view scrollbar flicker on mobile viewport (#204)

## Summary
- Change `overflow-y: auto` to `overflow: hidden` on `html`/`body` in
the mobile media query to prevent scrollbar flicker in the live view.

## Test plan
- [ ] Verify live view on viewport ≤ 1024px no longer shows scrollbar
flicker
- [ ] Confirm normal scrolling behavior is unaffected on larger
viewports

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk CSS-only change limited to the ≤1024px media query, but it
may affect expected scrolling behavior on small viewports if any content
needs to scroll.
> 
> **Overview**
> Updates the ≤1024px media query in `app.vue` to set `html, body` to
`overflow: hidden` (replacing `overflow-y: auto`) to prevent mobile
scrollbar flicker in the live view.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
abdcece. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Rafael <raf@kernel.sh>

Author: 3 people

images/chromium-headful/client/src/app.vue

@@ -127,7 +127,7 @@
   @media only screen and (max-width: 1024px) {
     html,
     body {
-      overflow-y: auto !important;
+      overflow: hidden !important;
       width: auto !important;
       height: auto !important;
     }