fix(desktop): 修复 10 个 agent 对话测试发现的缺陷

- BUG-001+002 P0: 模型选择器合并 SaaS 可用模型列表
- BUG-003 P1: 修复 relay 错误消息重复显示
- BUG-005 P1: 设置页面显示实际连接模式和地址
- BUG-006 P2: 统一 UI 语言为中文
- BUG-009 P3: 错误时隐藏建议按钮
- BUG-010 P3: 密码切换按钮添加 aria-label

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

desktop/src/store/chat/streamStore.ts

@@ -410,7 +410,7 @@ export const useStreamStore = create<StreamState>()(
             _chat?.updateMessages(msgs =>
               msgs.map(m =>
                 m.id === assistantId
-                  ? { ...m, content: `⚠️ ${error}`, streaming: false, error }
+                  ? { ...m, content: '', streaming: false, error }
                   : m.role === 'user' && m.optimistic && m.timestamp.getTime() >= streamStartTime
                     ? { ...m, optimistic: false }
                     : m