feat: add Native HEZ support for pool creation

assetConversion pallet requires pools to pair with Native token.

Changes:
- Added NATIVE_TOKEN_ID (-1) constant for relay chain HEZ
- Updated CreatePoolModal to use XCM location format for Native
- Pools can now be created: Native HEZ / PEZ, Native HEZ / wUSDT, etc.
- Fixed balance fetching for Native vs Asset tokens

web/src/types/dex.ts

@@ -11,4 +11,4 @@ export {
   TransactionStatus,
 } from '../../../shared/types/tokens';
 
-export { KNOWN_TOKENS, TOKEN_DISPLAY_SYMBOLS } from '../../../shared/constants';
+export { KNOWN_TOKENS, TOKEN_DISPLAY_SYMBOLS, NATIVE_TOKEN_ID } from '../../../shared/constants';