SocialsDialogViewModel.cs 515 -> 334. Extracted the DialogEntry record + the
row-level SocialSlotViewModel (service/logo/lock/edit/validate state, computed
Show* flags) into a new SocialSlotViewModel.cs partial. The dialog VM keeps the
sign-in gate, validation orchestration, and commit. DialogEntry stays public in
the same namespace.
Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 3 tracker updated in same commit.
