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.

cmd/server/tests/filter_builder_test.go

@@ -526,6 +526,16 @@ func TestFilterSuggestionsUnderRules(t *testing.T) {
 	assert.NotContains(t, valuesOf(results), "spacewestern", "zero-match values never offered")
 	assert.NotContains(t, valuesOf(results), "history", "values outside the rules never offered")
 
+	// Browse with NO other rules in place: the blank query still lists
+	// the field's values (the static similarity path would answer a
+	// single space with nothing). Regression for the empty-browse bug.
+	status, results = suggest("genre", " ", "")
+	require.Equal(t, http.StatusOK, status)
+	assert.Contains(t, valuesOf(results), "fantasy", "browse with no rules lists the field's values")
+	status, results = suggest("author", " ", "")
+	require.Equal(t, http.StatusOK, status)
+	assert.Contains(t, valuesOf(results), "Amara Voss", "author browse with no rules")
+
 	// Library field menu: visibility-scoped, id values + name display
 	// (this test's fixture libraries are named "Test Builder Sug …").
 	status, results = suggest("library", "Test Builder", "")