fix: update timer display color for completed state

Changed the timer display color from red to gray when the timer reaches zero, enhancing visual consistency with idle state. This adjustment aims to provide a calmer appearance when the timer completes.

Author: ryota-murakami

components/timer/TimerDisplay.tsx

@@ -36,7 +36,7 @@ export function TimerDisplay({
 
   // Determine color based on state
   const getColor = () => {
-    if (timeRemaining === 0) return '#EF4444' // red when complete
+    if (timeRemaining === 0) return '#9CA3AF' // gray when complete
     if (isPaused) return '#FBBF24' // amber when paused
     if (isRunning) return '#10B981' // green when running
     return '#9CA3AF' // gray when idle