fix: back button in subscriptions list uses correct callback

Changed callback_data from non-existent 'menu_main' to registered
'back_to_menu' handler so the back button actually returns to main menu.

app/handlers/promocode.py

@@ -148,7 +148,7 @@ async def process_promocode(message: types.Message, db_user: User, state: FSMCon
                     )
                 ]
             )
-        buttons.append([types.InlineKeyboardButton(text='❌ Отмена', callback_data='menu_main')])
+        buttons.append([types.InlineKeyboardButton(text='❌ Отмена', callback_data='back_to_menu')])
         await message.answer(
             texts.t(
                 'PROMOCODE_SELECT_SUBSCRIPTION',

app/handlers/subscription/my_subscriptions.py

@@ -101,7 +101,7 @@ def _build_subscriptions_keyboard(subscriptions: list, language: str) -> types.I
     # Back button
     buttons.append(
         [
-            types.InlineKeyboardButton(text='◀️ Назад', callback_data='menu_main'),
+            types.InlineKeyboardButton(text='◀️ Назад', callback_data='back_to_menu'),
         ]
     )
 
@@ -150,7 +150,7 @@ async def show_my_subscriptions(
         keyboard = types.InlineKeyboardMarkup(
             inline_keyboard=[
                 [types.InlineKeyboardButton(text='🛒 Купить подписку', callback_data='menu_buy')],
-                [types.InlineKeyboardButton(text='◀️ Назад', callback_data='menu_main')],
+                [types.InlineKeyboardButton(text='◀️ Назад', callback_data='back_to_menu')],
             ]
         )
     else: