feat(FN-5204): add test mode activation across settings, engine, and dashboard

- Add core settings schema/types support for testMode with model-resolution override handling
- Enforce engine session lane overrides in test mode with targeted helper coverage
- Add dashboard settings toggle plus persistent test-mode banner and related component tests
- Update settings documentation and parity/roundtrip tests for the new test mode behavior

packages/dashboard/app/styles.css

@@ -3189,6 +3189,13 @@ input[type="range"]:focus-visible {
   }
 }
 
+@media (min-width: 769px) and (max-width: 1024px) {
+  .board {
+    grid-template-columns: repeat(6, minmax(0, 1fr));
+    overflow-x: hidden;
+  }
+}
+
 /* === Mobile Responsive Overrides ===
    Cross-cutting mobile rules only. Component-specific overrides have been
    moved into co-located @media blocks at the bottom of each component's