feat(08-02): implement formatTimestamp utility

GREEN phase: Implement timestamp formatting with dayjs:
- Returns "Just now" for messages < 1 minute old
- Returns "X minutes ago" for messages < 1 hour old
- Returns "X hours ago" for messages < 24 hours old
- Returns "Yesterday" for messages 1 day old
- Returns day name (e.g., "Monday") for messages < 7 days old
- Returns MM/DD/YYYY format for older messages
- Handles invalid input gracefully (returns empty string)

All 6 tests passing.

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

jam-ui/src/utils/formatTimestamp.js

@@ -0,0 +1,33 @@
+import dayjs from 'dayjs';
+import relativeTime from 'dayjs/plugin/relativeTime';
+
+dayjs.extend(relativeTime);
+
+/**
+ * Formats a timestamp as relative time (e.g., "2 minutes ago", "Yesterday")
+ * @param {string} timestamp - ISO timestamp string
+ * @returns {string} Formatted relative time
+ */
+export const formatTimestamp = (timestamp) => {
+  if (!timestamp) return '';
+
+  try {
+    const date = dayjs(timestamp);
+    if (!date.isValid()) return '';
+
+    const now = dayjs();
+    const diffInMinutes = now.diff(date, 'minute');
+    const diffInHours = now.diff(date, 'hour');
+    const diffInDays = now.diff(date, 'day');
+
+    if (diffInMinutes < 1) return 'Just now';
+    if (diffInMinutes < 60) return `${diffInMinutes} minute${diffInMinutes > 1 ? 's' : ''} ago`;
+    if (diffInHours < 24) return `${diffInHours} hour${diffInHours > 1 ? 's' : ''} ago`;
+    if (diffInDays === 1) return 'Yesterday';
+    if (diffInDays < 7) return date.format('dddd'); // Day name (e.g., "Monday")
+    return date.format('MM/DD/YYYY');
+  } catch (error) {
+    console.error('formatTimestamp error:', error);
+    return '';
+  }
+};