feat(chat): align activity timing UI with hover-only end timestamps (#557)

* feat(chat): add shared activity timestamp formatter

* feat(chat): pass assistant created timestamp to message body

* feat(chat): render footer timestamp beside elapsed time

* feat(chat): pass timing metadata in reasoning text blocks

* feat(chat): propagate justification timing to timeline block

* feat(chat): show thinking duration with hover end time

* feat(chat): reveal tool end time on activity hover

packages/ui/src/components/chat/message/timeFormat.ts

@@ -0,0 +1,34 @@
+const pad2 = (value: number): string => String(value).padStart(2, '0');
+
+const isSameDay = (left: Date, right: Date): boolean => {
+    return (
+        left.getFullYear() === right.getFullYear() &&
+        left.getMonth() === right.getMonth() &&
+        left.getDate() === right.getDate()
+    );
+};
+
+const isValidTimestamp = (timestamp: number): boolean => {
+    return Number.isFinite(timestamp) && !Number.isNaN(new Date(timestamp).getTime());
+};
+
+export const formatTimestampForDisplay = (timestamp: number): string => {
+    if (!isValidTimestamp(timestamp)) {
+        return '';
+    }
+
+    const date = new Date(timestamp);
+    const now = new Date();
+
+    const timePart = `${pad2(date.getHours())}:${pad2(date.getMinutes())}:${pad2(date.getSeconds())}`;
+
+    if (isSameDay(date, now)) {
+        return timePart;
+    }
+
+    const yearPart = String(date.getFullYear()).slice(-2);
+    const monthPart = pad2(date.getMonth() + 1);
+    const dayPart = pad2(date.getDate());
+
+    return `${yearPart}-${monthPart}-${dayPart} ${timePart}`;
+};