Address PR review: combobox field layout and dark-mode contrast

- Wire the long-defined-but-unused _FIELD_CLASS into the container so pills and
  the search input form a single padded flex row; the flex-1 input now fills the
  widget instead of looking unclickable inside a larger box (affects both
  SearchSelect and FilterSelect via the shared shell).
- Filter option labels get text-body so they're readable on dark backgrounds.
- Filter +/- buttons get text-body (readable at rest) and hover:border-brand-strong
  so the border stays visible against the brand hover fill.
- Mirror the filter class changes in search_select.js and rebuild base.css.

https://claude.ai/code/session_01XzhXvMvw42CQGc9kmin3GS

games/static/base.css

@@ -2940,10 +2940,10 @@
       }
     }
   }
-  .hover\:border-brand {
+  .hover\:border-brand-strong {
     &:hover {
       @media (hover: hover) {
-        border-color: var(--color-brand);
+        border-color: var(--color-brand-strong);
       }
     }
   }