fix(frontend): post-review fixes sprint 2

- MitreTechniquePicker: guard sync effect while dropdown open to prevent
  mid-stroke wipe when onChange(null,null) propagates back as null props
- SimulationList: replace window.location.href with navigate() to keep
  TanStack Query cache intact on row click
- SimulationFormPage: simplify redundant ternary on SOC form onSubmit
- SimulationFormPage: remove dead .replace(' ','T') on executed_at
  (backend already returns ISO 8601 with T separator)

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

frontend/src/components/MitreTechniquePicker.tsx

@@ -37,12 +37,16 @@ export function MitreTechniquePicker({
   const debounceRef = useRef<ReturnType<typeof setTimeout> | null>(null);
   const containerRef = useRef<HTMLDivElement>(null);
   const listRef = useRef<HTMLUListElement>(null);
+  // True once we've synced the first real techniqueId from props (parent/API load).
+  // After that we stop reacting to null, so keystrokes that emit onChange(null,null)
+  // don't propagate back and wipe the input mid-stroke.
+  const hasHydratedFromProps = useRef(false);
 
-  // Sync display when selection changes from outside (initial load)
   useEffect(() => {
     if (techniqueId && techniqueName) {
       setInputValue(`${techniqueId} — ${techniqueName}`);
-    } else if (!techniqueId) {
+      hasHydratedFromProps.current = true;
+    } else if (!techniqueId && !hasHydratedFromProps.current) {
       setInputValue('');
     }
   }, [techniqueId, techniqueName]);