style: polish investigation workspace

- Summary panel: hide meaningless metrics (questions answered/remaining) until genuinely in progress; remove placeholder timestamps
- Understanding card: increased visual importance via larger heading, lighter border, more padding
- History section: reduced labels to brief forms ('History', 'Situation'), removed uppercase decorative labels from headings
- Investigation Map Preview: lighter borders, muted text, subtle background to signal provisional state
- Turn history cards: removed redundant subheadings ('Your answer', 'What changed') and divider lines
- Button label: 'Update situation' → 'Update'; padding consistent with design tokens
- Condition clarity fix: '!hasSelectedQuestion === false' → 'hasSelectedQuestion'
- Workspace polish section added to UX guidelines

components/investigation-map.jsx

@@ -82,8 +82,8 @@ export default function InvestigationMap({ turnCount = 0 }) {
   if (!hasActiveTopics && groups.established.length === 0) return null;
 
   return (
-    <div className="rounded-lg border border-gray-200 bg-white p-5" role="region" aria-label="Investigation map preview">
-      <h2 className="mb-1 text-sm font-semibold uppercase tracking-wide text-gray-500">
+    <div className="rounded-lg border border-gray-100 bg-gray-50/50 p-4" role="region" aria-label="Investigation map preview">
+      <h2 className="mb-1 text-sm font-semibold tracking-tight text-gray-400">
         Investigation Map Preview
       </h2>
       <p className="mb-3 text-xs text-gray-400">