refactor(ui): standardize user variable naming and avoid a SQL query when only userID is used

- Use `user` everywhere, instead of sometimes `loggedUser`
- Delay the instantiation of some variables: no need to perform SQL queries for
  nothing.
- Remove a SQL query getting the whole user struct when only user.ID is used.

internal/ui/oauth2_redirect.go

@@ -15,8 +15,6 @@ import (
 )
 
 func (h *handler) oauth2Redirect(w http.ResponseWriter, r *http.Request) {
-	sess := session.New(h.store, request.SessionID(r))
-
 	provider := request.RouteStringParam(r, "provider")
 	if provider == "" {
 		slog.Warn("Invalid or missing OAuth2 provider")
@@ -36,6 +34,7 @@ func (h *handler) oauth2Redirect(w http.ResponseWriter, r *http.Request) {
 
 	auth := oauth2.GenerateAuthorization(authProvider.GetConfig())
 
+	sess := session.New(h.store, request.SessionID(r))
 	sess.SetOAuth2State(auth.State())
 	sess.SetOAuth2CodeVerifier(auth.CodeVerifier())