Enable command queueing and fix message duplication issues

- Remove sending() state blocking to allow queueing multiple messages
- Add optimistic message creation with temp IDs for immediate UI feedback
- Implement QUEUED badge display matching TUI behavior (accent color, bold)
- Compute queued state: user message ID > last assistant message ID
- Clear prompt input immediately before async send for better UX
- Fix duplicate messages by properly finding and replacing temp messages
- Fix duplicate parts by clearing optimistic parts when real message arrives
- Fix state mutation bugs in message.part.updated handler (immutable updates)
- Fix state mutation bugs in message.updated handler (immutable updates)

Matches TUI implementation for consistent user experience across clients.

src/index.css

@@ -189,9 +189,22 @@ body {
 }
 
 .message-content {
-  display: flex;
-  flex-direction: column;
-  gap: 8px;
+  padding-top: 6px;
+  line-height: 1.6;
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+
+.message-queued-badge {
+  display: inline-block;
+  background-color: var(--accent-color);
+  color: white;
+  font-weight: bold;
+  padding: 4px 12px;
+  border-radius: 4px;
+  margin-bottom: 12px;
+  font-size: 12px;
+  letter-spacing: 0.5px;
 }
 
 .message-text {