Stop the input sheet covering the whole panel

`hidden` is only a UA stylesheet rule, so .sheet-wrap's display:flex beat
it and the sheet was never hidden: a full-screen scrim sat over the app
from the moment it loaded, blurring it and swallowing every tap, with an
empty option list because openSheet() had never run to fill it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

static/panel.css

@@ -17,6 +17,11 @@
 
 * { box-sizing: border-box; }
 
+/* Must beat every display rule below it. `hidden` is a plain UA style, so
+   any author rule -- .sheet-wrap's display:flex, say -- silently wins, and
+   an overlay that never hides sits over the whole panel eating taps. */
+[hidden] { display: none !important; }
+
 html { background: var(--bg); }
 
 body {