feat(groups): add Claude Code client restriction and session isolation

- Add claude_code_only field to restrict groups to Claude Code clients only
- Add fallback_group_id for non-Claude Code requests to use alternate group
- Implement ClaudeCodeValidator for User-Agent detection
- Add group-level session binding isolation (groupID in Redis key)
- Prevent cross-group sticky session pollution
- Update frontend with Claude Code restriction controls

backend/internal/pkg/ctxkey/ctxkey.go

@@ -7,4 +7,6 @@ type Key string
 const (
 	// ForcePlatform 强制平台（用于 /antigravity 路由），由 middleware.ForcePlatform 设置
 	ForcePlatform Key = "ctx_force_platform"
+	// IsClaudeCodeClient 是否为 Claude Code 客户端，由中间件设置
+	IsClaudeCodeClient Key = "ctx_is_claude_code_client"
 )