feat(chat): add multi-model comparison mode (M9.1)

Split-screen comparison of two AI models streaming simultaneously.
Toggle via header button. Desktop shows side-by-side panes, mobile
shows swipeable tabs. Uses streamWithModel() for provider-agnostic
parallel streaming.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/composables/useAI.ts

@@ -326,6 +326,32 @@ async function generateAutoTitle(conversationId: string) {
   }
 }
 
+/** Stream a specific provider/model — used by comparison mode */
+export async function streamWithModel(
+  provider: string,
+  model: string,
+  messages: { role: string; content: string }[],
+  onToken: (text: string) => void,
+  onError: (err: string) => void,
+  signal: AbortSignal,
+): Promise<void> {
+  const history = messages.map(m => ({ role: m.role as 'user' | 'assistant', content: m.content }))
+  const savedModel = activeModel.value
+  activeModel.value = model
+
+  try {
+    if (provider === 'claude') {
+      await streamClaude(history, onToken, onError, 'You are a helpful assistant.', false, signal)
+    } else if (provider === 'openrouter') {
+      await streamOpenRouter(history, onToken, onError, 'You are a helpful assistant.', signal)
+    } else {
+      await streamMock(history, onToken, signal)
+    }
+  } finally {
+    activeModel.value = savedModel
+  }
+}
+
 export function useAI() {
   const chatStore = useChatStore()