v6: Fix doc explorer search popover shadow in light theme (#4417)

## Summary

The doc explorer's search input and results popover had their shadow
hardcoded to `0 4px 12px oklch(0% 0 0 / 0.3)`. That's the dark-theme
value of `--shadow-popover`, pasted in as a literal instead of a
reference, and it's missing the token's second, subtler shadow layer
too. Every other popover in the app (dialog, dropdown, tooltip) reads
`var(--shadow-popover)` and re-themes on its own, so in light theme this
one was casting a shadow roughly four times heavier than its siblings.
Both spots now read the token directly.

## Test plan

- [x] Switch to light theme, open the doc explorer, and focus the search
box. The active-state input and the results popover both cast a soft,
light-appropriate shadow, not a heavy dark one.
- [x] Switch to dark theme and confirm the search popover shadow still
matches other popovers (dialog, dropdown, tooltip).

Refs: #4219

Author: trevor-scheer

packages/graphiql-plugin-doc-explorer/src/components/search.css

@@ -7,7 +7,7 @@
     & .graphiql-doc-explorer-search-input {
       background: oklch(var(--bg-canvas));
       border: 1px solid oklch(var(--border-default));
-      box-shadow: 0 4px 12px oklch(0% 0 0 / 0.3);
+      box-shadow: var(--shadow-popover);
     }
   }
 }
@@ -46,7 +46,7 @@
   background-color: oklch(var(--bg-canvas));
   border: 1px solid oklch(var(--border-default));
   border-radius: var(--border-radius-4);
-  box-shadow: 0 4px 12px oklch(0% 0 0 / 0.3);
+  box-shadow: var(--shadow-popover);
   max-height: 400px;
   overflow-y: auto;
   margin: 0;