fix(dashboard): show full name on welcome, fix gender mismatch on trial-expired card

User reported single-letter welcome "Добро пожаловать, О!" (short first_name
"Олег"-style users) and a confused "Истекла" feminine label on the masculine
"Пробный период истёк" trial-expired card.

* Add `src/utils/displayName.ts` helper that composes `first_name + last_name`
  with `username` and `#telegram_id` fallbacks. Single source of truth for
  user-facing name rendering across the app.
* Apply `displayName(user)` in Dashboard welcome, AppHeader mobile drawer,
  and DesktopSidebar profile chip. Now a user with `first_name="О"` and
  `last_name="Иванов"` sees "О Иванов" instead of just "О".
* `SubscriptionCardExpired` — context-aware Russian label: for trial
  subscriptions render masculine "Истёк" (agrees with "пробный период"),
  for paid subscriptions keep feminine "Истекла" (agrees with "подписка").
  Uses i18next `context: subscription.is_trial ? 'trial' : ''` — falls back
  to base key for EN/ZH/FA which are grammatically neutral.
* Add `expiredDate_trial: "Истёк"` only to `ru.json` (no changes needed for
  other locales — i18next context falls back to `expiredDate`).

src/pages/Dashboard.tsx

@@ -3,6 +3,7 @@ import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { Link, useNavigate } from 'react-router';
 import { useTranslation } from 'react-i18next';
 import { useAuthStore } from '../store/auth';
+import { displayName } from '../utils/displayName';
 import { useBlockingStore } from '../store/blocking';
 import { subscriptionApi } from '../api/subscription';
 import { referralApi } from '../api/referral';
@@ -254,7 +255,7 @@ export default function Dashboard() {
       {/* Header */}
       <div data-onboarding="welcome">
         <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">
-          {t('dashboard.welcome', { name: user?.first_name || user?.username || '' })}
+          {t('dashboard.welcome', { name: displayName(user) })}
         </h1>
         <div className="mt-1 flex flex-wrap items-center gap-2">
           <p className="text-dark-400">{t('dashboard.yourSubscription')}</p>