Refactor for GRAM (ex TON) integration

- Updated imports and references from TON to GRAM across multiple modules.
- Renamed functions and variables to reflect the change from TON to GRAM.
- Adjusted validation checks for top-up and recharge amounts to use GRAM limits.
- Modified error messages and exceptions to indicate GRAM instead of TON.
- Updated tests to ensure they reflect the new GRAM terminology and functionality.
- Introduced new constants for GRAM-related configurations.

CONTRIBUTING.md

@@ -30,7 +30,7 @@ pyfragment/
   client.py           — FragmentClient (public entry point)
   core/               — transport, cookies, constants
   domains/            — one package per feature domain
-    ads/              — recharge_ads, topup_ton
+    ads/              — recharge_ads, topup_gram
     anonymous_numbers/— buy_number, manage_number
     giveaways/        — giveaway_stars, giveaway_premium
     marketplace/      — search_usernames, search_numbers, search_gifts