feat(KB-061): add model filter search to model selectors

- Add search input with clear button and results count to ModelSelectorTab
- Extract filterModels utility for shared filtering logic across components
- Apply filtering to SettingsModal model section
- Add CSS styles for filter UI components
- Add comprehensive tests for ModelSelectorTab and SettingsModal filtering

packages/dashboard/app/utils/modelFilter.ts

@@ -0,0 +1,15 @@
+import type { ModelInfo } from "../api";
+
+/**
+ * Filter models by search terms matching provider, id, or name.
+ * Supports multi-word filters (space-separated AND logic).
+ * Case-insensitive substring matching.
+ */
+export function filterModels(models: ModelInfo[], filter: string): ModelInfo[] {
+  const terms = filter.toLowerCase().trim().split(/\s+/).filter(Boolean);
+  if (terms.length === 0) return models;
+  return models.filter((m) => {
+    const haystack = `${m.provider} ${m.id} ${m.name}`.toLowerCase();
+    return terms.every((term) => haystack.includes(term));
+  });
+}