refactor(store): single account name on users; rename profile/laps tables

users.display_name becomes users.name and is now the only human-facing
name -- profiles.name is dropped (it duplicated the account name; the
Profile page's Name field now edits users.name through PUT /api/profile,
which carries Name alongside the profiles columns). The profile table
becomes profiles and laps becomes activity_laps, homogeneous with
activity_samples. Onboarding pre-fills the display name from the OIDC
claim's name.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/api/session.go

@@ -114,7 +114,7 @@ func (s *Server) handleSessionMe(w http.ResponseWriter, r *http.Request) {
 	resp := sessionMeResponse{Name: claims.Name, Email: claims.Email}
 	if u, found := userFromContext(r.Context()); found {
 		resp.HasProfile = true
-		resp.DisplayName = u.DisplayName
+		resp.DisplayName = u.Name
 		profile, err := s.DB.GetProfile(r.Context(), u.ID)
 		if err != nil {
 			writeError(w, http.StatusInternalServerError, err.Error())