fix(trajectory): show exact millisecond durations consistently

Drop the details-panel Duration toggle: Duration rows always show
integer milliseconds, matching the cell time column. Timeline labels
(Total/TTFT/Decoding) and step-group descriptions previously fell back
to second labels at or above one second; they now also show exact
milliseconds via the shared formatDurationMillis formatter.

packages/client/ui-trajectory/tests/layout.spec.tsx

@@ -207,7 +207,7 @@ describe('deriveTrajectoryLayout', () => {
       },
     ] as unknown as ConversationSnapshot['nodes']
     const turns = deriveTrajectoryLayout({ codeDispatches: new Map(), nodes, partial: null, runningCalls: [] })
-    expect(turns[0]?.groups[0]?.description).toBe('3 s bash×2')
+    expect(turns[0]?.groups[0]?.description).toBe('3,000 ms bash×2')
   })
 
   it('assigns each user message to its enclosing turn instead of pooling into Turn 1', () => {