feat(frontend): 支持 Gemini OAuth 类型选择 (Code Assist/AI Studio)

- CreateAccountModal.vue: 新增 OAuth 类型选择 UI
- ReAuthAccountModal.vue: 重授权支持选择类型
- OAuthAuthorizationFlow.vue: 新增 Project ID 输入框
- AccountTestModal.vue: Gemini 模型默认选择优化
- useGeminiOAuth.ts: OAuth 逻辑参数变更
- gemini.ts: API 调用更新

frontend/src/components/account/AccountTestModal.vue

@@ -329,11 +329,18 @@ const loadAvailableModels = async () => {
   selectedModelId.value = '' // Reset selection before loading
   try {
     availableModels.value = await adminAPI.accounts.getAvailableModels(props.account.id)
-    // Default to first model (usually Sonnet)
+    // Default selection by platform
     if (availableModels.value.length > 0) {
-      // Try to select Sonnet as default, otherwise use first model
-      const sonnetModel = availableModels.value.find((m) => m.id.includes('sonnet'))
-      selectedModelId.value = sonnetModel?.id || availableModels.value[0].id
+      if (props.account.platform === 'gemini') {
+        const preferred =
+          availableModels.value.find((m) => m.id === 'gemini-2.5-pro') ||
+          availableModels.value.find((m) => m.id === 'gemini-3-pro')
+        selectedModelId.value = preferred?.id || availableModels.value[0].id
+      } else {
+        // Try to select Sonnet as default, otherwise use first model
+        const sonnetModel = availableModels.value.find((m) => m.id.includes('sonnet'))
+        selectedModelId.value = sonnetModel?.id || availableModels.value[0].id
+      }
     }
   } catch (error) {
     console.error('Failed to load available models:', error)