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/utils/displayName.ts

@@ -0,0 +1,23 @@
+/**
+ * Composes a user-facing display name from first_name + last_name with sensible fallbacks.
+ *
+ * Why: single-letter first_name (e.g., "О") looked confusing alone ("Добро пожаловать, О!").
+ * Combining with last_name makes truncated/short first names readable.
+ */
+export interface NameSource {
+  first_name?: string | null;
+  last_name?: string | null;
+  username?: string | null;
+  telegram_id?: number | null;
+}
+
+export function displayName(user?: NameSource | null): string {
+  if (!user) return '';
+  const fullName = [user.first_name, user.last_name]
+    .filter((part): part is string => Boolean(part && part.trim()))
+    .join(' ');
+  if (fullName) return fullName;
+  if (user.username) return user.username;
+  if (user.telegram_id) return `#${user.telegram_id}`;
+  return '';
+}