fix(env): stabilize .env parsing and add feature toggles

- Handle dotenv placeholders like 'KEY=  # comment' without validation crashes

- Add REQUIRED_CHANNEL_SUBSCRIBE_TO_USE to explicitly enable channel gate

- Add REFERRAL_ENABLED to fully disable referral flow and bonuses

- Hide referral UI/actions when disabled and ignore ref start params

- Update .env.example defaults and README docs for new flags

bot/keyboards/inline/user_keyboards.py

@@ -28,13 +28,16 @@ def get_main_menu_inline_keyboard(
         )
     )
 
-    referral_button = InlineKeyboardButton(
-        text=_(key="menu_referral_inline"),
-        callback_data="main_action:referral")
     promo_button = InlineKeyboardButton(
         text=_(key="menu_apply_promo_button"),
         callback_data="main_action:apply_promo")
-    builder.row(referral_button, promo_button)
+    if settings.REFERRAL_ENABLED:
+        referral_button = InlineKeyboardButton(
+            text=_(key="menu_referral_inline"),
+            callback_data="main_action:referral")
+        builder.row(referral_button, promo_button)
+    else:
+        builder.row(promo_button)
 
     language_button = InlineKeyboardButton(
         text=_(key="menu_language_settings_inline"),