Make the hidden attribute win over display rules

.drop-overlay sets display: flex, which beats the UA's [hidden] rule, so
the "Drop to analyse" overlay covered the page permanently and hid the
status line. A global [hidden] { display: none !important } fixes it and
any other element that pairs `hidden` with a display class.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015UXA39Sh9coAx4FhRkd8R2

static/app.css

@@ -20,6 +20,11 @@
 
 * { box-sizing: border-box; }
 
+/* The UA's [hidden] { display: none } loses to any class that sets display
+   (e.g. .drop-overlay's flex), which left the overlay permanently covering the
+   page. Make the attribute win everywhere. */
+[hidden] { display: none !important; }
+
 html, body {
   margin: 0;
   height: 100%;