Auth: nedokoncena registracia sa da dokoncit novym QR kodom

Ucet bez potvrdeneho kodu (auth_token is NULL a totp_last_step == 0)
uz neblokuje meno: opakovany register_account vyda novy secret (stary
QR prestane platit) a login vyhodi RegistrationIncomplete, na ktoru
server odpovie novym QR -- klient sa prepne na registracny tab.
Admin statistiky vykazuju nedokoncene registracie osobitne, Hraci
celkom pocita len potvrdene ucty. Novy event register v analytike.

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

frontend/src/pages/admin/AdminStats.tsx

@@ -20,6 +20,7 @@ interface DailyStats {
   completion_rate: number | null;
   avg_game_duration_minutes: number | null;
   total_players: number;
+  unconfirmed_players: number;
   peak_hours: Record<string, number>;
   rounds_per_day: Record<string, number>;
   pageviews_per_day: Record<string, number>;
@@ -155,6 +156,7 @@ export default function AdminStats() {
     <div className="flex flex-col gap-4">
       <div className={`flex gap-3 ${desktop ? '' : 'flex-wrap'}`}>
         <SummaryCard label="Hráči celkom" value={String(data.total_players)} />
+        <SummaryCard label="Nedokončené registrácie" value={String(data.unconfirmed_players)} />
         <SummaryCard
           label="Dokončené hry"
           value={data.completion_rate != null ? `${Math.round(data.completion_rate * 100)}%` : '—'}