fix(dashboard): address PR review feedback for model filtering and insights

- Always apply configured-provider filter (even when empty set)
- Convert getConfiguredProviderNames to async (fs/promises)
- Validate model overrides as non-empty provider+model pair
- Clear stale persisted insight model when no longer available
- Replace hardcoded max-height with design token in overflow menu

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/dashboard/app/components/Header.css

@@ -702,7 +702,7 @@
   border-radius: var(--radius-md);
   box-shadow: var(--shadow-lg);
   min-width: 140px;
-  max-height: min(400px, 70vh);
+  max-height: min(70vh, calc(var(--space-2xl) * 17));
   overflow-y: auto;
   z-index: 200;
   padding: var(--space-xs) 0;