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/components/dashboard/SubscriptionCardExpired.tsx

@@ -228,7 +228,11 @@ export default function SubscriptionCardExpired({
       >
         <div className="flex items-center">
           <div className="mb-0.5 font-mono text-[10px] font-medium uppercase tracking-wider text-dark-50/30">
-            {isLimited ? t('dashboard.expired.activeUntil') : t('dashboard.expired.expiredDate')}
+            {isLimited
+              ? t('dashboard.expired.activeUntil')
+              : t('dashboard.expired.expiredDate', {
+                  context: subscription.is_trial ? 'trial' : '',
+                })}
           </div>
           <div className="ml-3 text-base font-bold tracking-tight text-dark-50/50">
             {formattedDate}