feat: add message timestamps with smart formatting

- Added timestamp field to Message struct in Rust backend
- Timestamps automatically captured on message creation
- Smart relative time formatting (Just now, Xm ago, time, date)
- Timestamps display below message content with subtle styling
- Fixed avatar squishing issue with flex-shrink: 0
- Backward compatible with existing messages via serde(default)

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

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

src/styles.css

@@ -86,6 +86,7 @@ body {
   border: 1px solid var(--border);
   background-size: cover;
   background-position: center;
+  flex-shrink: 0;
 }
 
 .avatar-circle-large {
@@ -287,6 +288,24 @@ body {
   margin-top: 12px;
 }
 
+/* Message timestamp */
+.message-timestamp {
+  font-size: 10px;
+  color: var(--text-secondary);
+  opacity: 0.6;
+  margin-top: 4px;
+  user-select: none;
+}
+
+.message.user .message-timestamp {
+  text-align: right;
+}
+
+.message.assistant .message-timestamp {
+  text-align: left;
+  margin-left: 4px;
+}
+
 /* Message action buttons */
 .message-actions {
   position: absolute;