fix: Dashboard DetailPanel uses flat scrollable layout matching mock

Changed from tabs-based to children-based DetailPanel layout:
- Flat scrollable sections: Open Details → Overview → Errors → Route Flow → Processor Timeline
- Title shows "route — exchangeId" matching mock pattern
- Removed unused state (detailTab, processorIdx)
- Added panelSectionMeta CSS for duration display in timeline header

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Dashboard/Dashboard.module.css

@@ -58,6 +58,18 @@
   letter-spacing: 0.5px;
   color: var(--text-muted);
   margin-bottom: 10px;
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+}
+
+.panelSectionMeta {
+  font-size: 11px;
+  font-weight: 400;
+  text-transform: none;
+  letter-spacing: 0;
+  color: var(--text-muted);
+  font-family: var(--font-mono);
 }
 
 .overviewGrid {