fix(frontend): 修复账号管理页面 API Key 类型的提示文案错误

- 添加 OpenAI/Gemini 平台的 baseUrlHint 和 apiKeyHint 国际化文案
- 修改 CreateAccountModal 和 EditAccountModal 根据平台显示正确提示
- 将重复的平台判断逻辑抽取为 computed 属性，优化代码结构

frontend/src/i18n/locales/en.ts

@@ -915,6 +915,11 @@ export default {
       apiKeyRequired: 'API Key *',
       apiKeyPlaceholder: 'sk-ant-api03-...',
       apiKeyHint: 'Your Claude Console API Key',
+      // OpenAI specific hints
+      openai: {
+        baseUrlHint: 'Leave default for official OpenAI API',
+        apiKeyHint: 'Your OpenAI API Key'
+      },
       modelRestriction: 'Model Restriction (Optional)',
       modelWhitelist: 'Model Whitelist',
       modelMapping: 'Model Mapping',
@@ -1076,6 +1081,7 @@ export default {
         modelPassthrough: 'Gemini Model Passthrough',
         modelPassthroughDesc:
           'All model requests are forwarded directly to the Gemini API without model restrictions or mappings.',
+        baseUrlHint: 'Leave default for official Gemini API',
         apiKeyHint: 'Your Gemini API Key (starts with AIza)'
       },
       // Re-Auth Modal

frontend/src/i18n/locales/zh.ts

@@ -1054,6 +1054,11 @@ export default {
       apiKeyRequired: 'API Key *',
       apiKeyPlaceholder: 'sk-ant-api03-...',
       apiKeyHint: '您的 Claude Console API Key',
+      // OpenAI specific hints
+      openai: {
+        baseUrlHint: '留空使用官方 OpenAI API',
+        apiKeyHint: '您的 OpenAI API Key'
+      },
       modelRestriction: '模型限制（可选）',
       modelWhitelist: '模型白名单',
       modelMapping: '模型映射',
@@ -1197,7 +1202,8 @@ export default {
       gemini: {
         modelPassthrough: 'Gemini 直接转发模型',
         modelPassthroughDesc: '所有模型请求将直接转发至 Gemini API，不进行模型限制或映射。',
-        apiKeyHint: 'Your Gemini API Key（以 AIza 开头）'
+        baseUrlHint: '留空使用官方 Gemini API',
+        apiKeyHint: '您的 Gemini API Key（以 AIza 开头）'
       },
       // Re-Auth Modal
       reAuthorizeAccount: '重新授权账号',