fix(filters): browse with no rules works, popup under the input, no sticky headers (user report)

- Blank typed queries rode the STATIC autocomplete path for the classic
  six fields, where a single space similarity-matches into nothing —
  clicking an empty value box with no other filters said 'No matching
  values' in a library full of values. Blank = browse now always rides
  the composed engine (which trims and answers the top values); static
  keeps its pinned contract for real typed queries. Regression-pinned:
  browse with no rules lists the field's values.
- The value popup now drops BELOW the input (top: 100%), not over it.
- Field-menu group headers lost position:sticky — the search box alone
  stays pinned.

web/static/input.css

@@ -1962,6 +1962,8 @@
   .fb-menu-value {
     width: 16rem;
     left: 0;
+    top: 100%;
+    margin-top: 0.25rem;
   }
   .fb-menu-group {
     font-size: 0.65rem;
@@ -1974,9 +1976,6 @@
     background-color: color-mix(in srgb, var(--text-primary) 5%, transparent);
     border-top: 1px solid var(--border);
     border-bottom: 1px solid var(--border);
-    position: sticky;
-    top: 0;
-    z-index: 1;
   }
   .fb-menu-filter-wrap {
     padding: 0.4rem 0.5rem;