feat(antigravity): 添加混合调度可选功能

- 后端：账户模型添加 IsMixedSchedulingEnabled() 方法，读取 extra.mixed_scheduling
- 后端：gateway_service 和 gemini_messages_compat_service 支持混合调度逻辑
- 后端：分组创建支持指定 platform 参数
- 前端：账户创建/编辑弹窗添加混合调度开关（仅 antigravity 账户显示）
- 前端：混合调度开关添加问号图标和 tooltip 说明
- 前端：GroupSelector 支持根据 mixedScheduling 属性过滤分组
- 前端：分组创建支持选择 platform
- 测试：e2e 测试添加 ENDPOINT_PREFIX 环境变量支持混合/隔离模式测试
- 测试：删除过时的 Claude signature 测试用例

frontend/src/i18n/locales/en.ts

@@ -940,6 +940,10 @@ export default {
       priority: 'Priority',
       priorityHint: 'Higher priority accounts are used first',
       higherPriorityFirst: 'Higher value means higher priority',
+      mixedScheduling: 'Mixed Scheduling',
+      mixedSchedulingHint: 'Enable to participate in Anthropic/Gemini group scheduling',
+      mixedSchedulingTooltip:
+        'When enabled, this account can be scheduled by /v1/messages and /v1beta endpoints. Otherwise, it will only be scheduled by /antigravity. Note: Anthropic Claude and Antigravity Claude cannot be mixed in the same context. Please manage groups carefully when enabled.',
       creating: 'Creating...',
       updating: 'Updating...',
       accountCreated: 'Account created successfully',