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;

src/GmRelay.Web/Services/SessionService.cs

@@ -2707,6 +2707,7 @@ public sealed class SessionService(
     public async Task LeaveClubMembershipAsync(Guid membershipId, Guid playerId)
     {
         await using var conn = await dataSource.OpenConnectionAsync();
+        // Active membership: withdraw by setting status = 'Left'.
         var rows = await conn.ExecuteAsync(
             """
             UPDATE club_memberships
@@ -2714,9 +2715,22 @@ public sealed class SessionService(
             WHERE id = @MembershipId AND player_id = @PlayerId AND status = 'Active'
             """,
             new { MembershipId = membershipId, PlayerId = playerId });
-        if (rows == 0)
+        if (rows > 0)
         {
-            throw new InvalidOperationException($"Active membership {membershipId} not found for player.");
+            return;
+        }
+
+        // Pending application: cancel by setting status = 'Rejected' so the user can re-apply later.
+        var cancelled = await conn.ExecuteAsync(
+            """
+            UPDATE club_memberships
+            SET status = 'Rejected', decided_at = now()
+            WHERE id = @MembershipId AND player_id = @PlayerId AND status = 'Pending'
+            """,
+            new { MembershipId = membershipId, PlayerId = playerId });
+        if (cancelled == 0)
+        {
+            throw new InvalidOperationException($"Membership {membershipId} is not Active or Pending for this player.");
         }
     }