fix(filters): value menu flash-on-click, empty state, sticky field filter (user report)

- The flash was a race: on localhost the suggestion fetch resolved
  between the focus (mousedown) and the click event, so the just-opened
  menu was closed by its own click-outside handler — the input sits
  outside the menu div. Clicks anywhere inside the value cell now
  never close the menu; outside clicks do.
- The menu opens immediately in a loading state; an empty browse
  renders 'No matching values' instead of nothing. Plain free-input
  fields (page_count etc.) open no menu at all.
- Field menu: the filter input moved outside the scroll region — it
  sticks while the grouped list scrolls beneath it.

web/static/input.css

@@ -2085,3 +2085,24 @@
     opacity: 0.7;
     cursor: default;
   }
+  /* Field menu: the filter input lives OUTSIDE the scroll region so it
+     sticks while the groups scroll beneath it. */
+  .fb-menu-fields {
+    display: flex;
+    flex-direction: column;
+    max-height: 18rem;
+    overflow: hidden;
+  }
+  .fb-menu-fields .fb-menu-filter-wrap {
+    flex-shrink: 0;
+    border-bottom: 1px solid var(--border);
+  }
+  .fb-menu-fields .fb-menu-scroll {
+    overflow-y: auto;
+    min-height: 0;
+  }
+  .fb-value-cell .fb-menu-empty {
+    padding: 0.5rem 0.75rem;
+    font-size: 0.78rem;
+    color: var(--text-secondary);
+  }