Add user names, team-scoped sandbox guard, and login robustness fixes

- Add name column to users (migration + sqlc regen); propagate through JWT
  claims, auth context, all auth/OAuth handlers, service layer, and frontend
- Sidebar and team page show name instead of email; team page splits Name/Email
  into separate columns
- Block sandbox creation in UI and API when user has no active team context
- loginTeam helper falls back to first active team when no default is set,
  fixing login for invited users with no is_default membership
- Exclude soft-deleted teams from GetDefaultTeamForUser, GetBYOCTeams queries
- Guard host creation against soft-deleted teams in service/host.go
- SwitchTeam re-fetches name from DB instead of trusting stale JWT claim
- Reset teams store on login so stale data from a previous session never persists
- Update openapi.yaml: add name to SignupRequest and AuthResponse schemas

internal/api/handlers_sandbox.go

@@ -79,6 +79,10 @@ func (h *sandboxHandler) Create(w http.ResponseWriter, r *http.Request) {
 	}
 
 	ac := auth.MustFromContext(r.Context())
+	if ac.TeamID == "" {
+		writeError(w, http.StatusForbidden, "no_team", "no active team context; re-authenticate")
+		return
+	}
 
 	sb, err := h.svc.Create(r.Context(), service.SandboxCreateParams{
 		TeamID:     ac.TeamID,