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/handler/dto/mappers.go

@@ -85,6 +85,8 @@ func GroupFromServiceShallow(g *service.Group) *Group {
 		ImagePrice1K:     g.ImagePrice1K,
 		ImagePrice2K:     g.ImagePrice2K,
 		ImagePrice4K:     g.ImagePrice4K,
+		ClaudeCodeOnly:   g.ClaudeCodeOnly,
+		FallbackGroupID:  g.FallbackGroupID,
 		CreatedAt:        g.CreatedAt,
 		UpdatedAt:        g.UpdatedAt,
 		AccountCount:     g.AccountCount,