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`).
