feat(kanban): Add count-up timer component for in-progress cards

- Create CountUpTimer component that displays elapsed time in MM:SS format
- Timer starts counting from when a card moves to in_progress status
- Shows timer on running cards (purple) and idle in_progress cards (yellow)
- Updates every second to show current elapsed time

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/components/ui/count-up-timer.tsx

@@ -0,0 +1,63 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { Clock } from "lucide-react";
+
+interface CountUpTimerProps {
+  startedAt: string; // ISO timestamp string
+  className?: string;
+}
+
+/**
+ * Formats elapsed time in MM:SS format
+ * @param seconds - Total elapsed seconds
+ * @returns Formatted string like "00:00", "01:30", "59:59", etc.
+ */
+function formatElapsedTime(seconds: number): string {
+  const minutes = Math.floor(seconds / 60);
+  const remainingSeconds = seconds % 60;
+
+  const paddedMinutes = minutes.toString().padStart(2, "0");
+  const paddedSeconds = remainingSeconds.toString().padStart(2, "0");
+
+  return `${paddedMinutes}:${paddedSeconds}`;
+}
+
+/**
+ * CountUpTimer component that displays elapsed time since a given start time
+ * Updates every second to show the current elapsed time in MM:SS format
+ */
+export function CountUpTimer({ startedAt, className = "" }: CountUpTimerProps) {
+  const [elapsedSeconds, setElapsedSeconds] = useState(0);
+
+  useEffect(() => {
+    // Calculate initial elapsed time
+    const startTime = new Date(startedAt).getTime();
+
+    const calculateElapsed = () => {
+      const now = Date.now();
+      const elapsed = Math.floor((now - startTime) / 1000);
+      return Math.max(0, elapsed); // Ensure non-negative
+    };
+
+    // Set initial value
+    setElapsedSeconds(calculateElapsed());
+
+    // Update every second
+    const interval = setInterval(() => {
+      setElapsedSeconds(calculateElapsed());
+    }, 1000);
+
+    return () => clearInterval(interval);
+  }, [startedAt]);
+
+  return (
+    <div
+      className={`flex items-center gap-1 text-xs text-muted-foreground ${className}`}
+      data-testid="count-up-timer"
+    >
+      <Clock className="w-3 h-3" />
+      <span data-testid="timer-display">{formatElapsedTime(elapsedSeconds)}</span>
+    </div>
+  );
+}