chore(ui): remove dead code from navigation redesign

Deleted:
- ScopeTrail component (replaced by inline breadcrumb in TopBar)
- ExchangeList component (replaced by Dashboard DataTable)
- ExchangeDetail page (replaced by inline split view)

Removed from Dashboard:
- flattenProcessors() function (unused after detail panel removal)
- 11 dead CSS classes (panelSection, overviewGrid, errorBlock,
  inspectLink, openDetailLink, filterBar, etc.)

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

ui/src/pages/Dashboard/Dashboard.tsx

@@ -66,24 +66,6 @@ function durationClass(ms: number, status: string): string {
   return styles.durBreach
 }
 
-function flattenProcessors(nodes: any[]): any[] {
-  const result: any[] = []
-  let offset = 0
-  function walk(node: any) {
-    result.push({
-      name: node.processorId || node.processorType,
-      type: node.processorType,
-      durationMs: node.durationMs ?? 0,
-      status: node.status === 'COMPLETED' ? 'ok' : node.status === 'FAILED' ? 'fail' : 'ok',
-      startMs: offset,
-    })
-    offset += node.durationMs ?? 0
-    if (node.children) node.children.forEach(walk)
-  }
-  nodes.forEach(walk)
-  return result
-}
-
 // ─── Table columns (base, without inspect action) ────────────────────────────
 
 function buildBaseColumns(): Column<Row>[] {