fix(beacon): stop the processing overlay covering the page on load

beacon.html "just shows a spinner": the `data-ds="new"` redesign's
`.ds-overlay-backdrop { display: flex }` (an author rule) overrides the UA
`[hidden] { display: none }`, so the processing overlay — `hidden` in the markup
— rendered on load as a full-screen backdrop over the whole page. (Not the #28
watchdog, which is compiled into the bundle correctly.)

Add `.ds-overlay-backdrop[hidden] { display: none }` (specificity 0,2,0 beats the
base rule) so the `hidden` attribute actually hides it — the same guard the video
modal already uses. No JS change; the JS inline-style toggle still works. The
overlay now only shows while a torrent is being added. Rebuilt CSS + dist.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

css/tailwind.source.css

@@ -573,6 +573,13 @@
     -webkit-backdrop-filter: blur(var(--overlay-backdrop-blur));
   }
 
+  /* `display: flex` above overrides the UA `[hidden] { display: none }`, so an
+     overlay with the `hidden` attribute (e.g. the beacon processing spinner)
+     would render on load and cover the whole page. Restore hide-on-hidden. */
+  .ds-overlay-backdrop[hidden] {
+    display: none;
+  }
+
   .popularity-modal__backdrop {
     z-index: var(--z-overlay-popover);
     padding: var(--space-md);