fix(web): allow cancelling pending applications; drop contradictory message guard

Address review feedback from PR #119:

- LeaveClubMembershipAsync: was rejecting Pending rows because the SQL
  required status = 'Active', so clicking "Отозвать заявку" on a Pending
  membership surfaced a misleading "Active membership X not found"
  InvalidOperationException. Now the method first tries Active -> Left
  and falls back to Pending -> Rejected so the same UI flow covers both
  states.
- PublicClub.razor TrySubmitApplicationAsync: removed the empty-input
  guard that contradicted the "(необязательно)" label and the server
  side (AuthorizedMembershipService already trims and accepts null).

No tests broken (493 still passing), no public-API changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/GmRelay.Web/Components/Pages/PublicClub.razor

@@ -167,12 +167,6 @@ else if (club is not null)
         if (club is null)
             return;
 
-        if (string.IsNullOrWhiteSpace(applicationMessage))
-        {
-            applicationError = "Введите сообщение или оставьте поле пустым.";
-            return;
-        }
-
         try
         {
             isSubmittingApplication = true;