feat: add thinking indicator while model is generating

- src/types.ts: Add isThinking flag to ChatMessage to track transient
  'model is working' state.

- src/chat-view.ts: Set isThinking: true on the assistant placeholder
  message when user sends input. Clear it when the first stream chunk
  arrives or on error. Update render() to show a spinner + 'Thinking…'
  text while isThinking is active.

- styles.css: Add ollama-thinking-indicator class with a CSS spinner
  animation and muted italic text styling.

styles.css

@@ -149,3 +149,29 @@
 .ollama-send-button:active {
   filter: brightness(0.95);
 }
+
+/* Thinking indicator */
+.ollama-thinking-indicator {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--size-4-1);
+  font-style: italic;
+  color: var(--text-muted);
+}
+
+.ollama-thinking-indicator::before {
+  content: '';
+  display: inline-block;
+  width: 0.9em;
+  height: 0.9em;
+  border: 2px solid var(--text-muted);
+  border-top-color: transparent;
+  border-radius: 50%;
+  animation: ollama-spin 1s linear infinite;
+}
+
+@keyframes ollama-spin {
+  to {
+    transform: rotate(360deg);
+  }
+}