refactor: AI session managed server-side, sessionId returned from API

editor/components/AIPanel.vue

@@ -31,12 +31,12 @@ async function send() {
     return
   }
 
-  store.ensureAISession()
   messages.value.push({ role: 'user', content: msg })
   loading.value = true
 
   try {
-    const { result } = await sendAIRequest(store.aiSessionId, msg, mode.value, store.deepseekKey)
+    const { result, sessionId: newSid } = await sendAIRequest(msg, mode.value, store.deepseekKey, store.aiSessionId || undefined)
+    if (newSid) store.setAISessionId(newSid)
     messages.value.push({ role: 'assistant', content: mode.value === 'json' ? '已生成 JSON，请查看编辑器面板' : '代码已修改，请查看预览窗口' })
 
     if (mode.value === 'json') {
@@ -58,7 +58,7 @@ function onKeydown(e: KeyboardEvent) {
 }
 
 function newSession() {
-  store.newAISession()
+  store.clearAISession()
   messages.value = []
   errorMsg.value = ''
 }