fix(web): correct asset IDs/decimals + chain routing, string-BigInt amounts, presale signer

Fund-logic hardening (audit remediation):
- TransferModal: source asset id/decimals from the canonical MINTABLE_ASSETS
  (wUSDT=1000, wDOT=1001, wETH=1002, wBTC=1003) instead of the wrong hardcoded
  ids (BTC=3 was the deprecated "Old USDT", ETH=4, DOT=5). Route every non-native
  asset through the Asset Hub api; only native HEZ uses the relay balances pallet.
  Removes the latent wrong-asset / wrong-chain transfer.
- Replace BigInt(parseFloat(x)*10**dec) with string-based parseTokenInput in
  TransferModal + 3 DEX init modals; BigInt(float) threw RangeError on common
  fractional amounts (e.g. 100.3 HEZ), breaking sends and pool/bridge init.
- Presale.contribute now resolves a signer via getSigner and passes { signer }
  to signAndSend (was signer-less -> broken for all users).
- presale.ts contribute/refund/claimVested now reject on dropped/invalid/usurped/
  retracted tx states instead of hanging the UI forever.

web/src/pages/Presale.tsx

@@ -14,7 +14,7 @@ import { toast } from 'sonner';
 export default function Presale() {
   const { t } = useTranslation();
   const navigate = useNavigate();
-  const { api, selectedAccount, isApiReady } = usePezkuwi();
+  const { api, selectedAccount, isApiReady, walletSource } = usePezkuwi();
   const { balances } = useWallet();
 
   const [wusdtAmount, setWusdtAmount] = useState('');
@@ -107,9 +107,13 @@ export default function Presale() {
     try {
       const amountWithDecimals = Math.floor(amount * 1_000_000); // 6 decimals
 
+      // Get signer (extension or WalletConnect) — no global api.setSigner exists
+      const { getSigner } = await import('@/lib/get-signer');
+      const injector = await getSigner(selectedAccount.address, walletSource, api);
+
       const tx = api.tx.presale.contribute(amountWithDecimals);
 
-      await tx.signAndSend(selectedAccount.address, ({ status, events }) => {
+      await tx.signAndSend(selectedAccount.address, { signer: injector.signer }, ({ status, events }) => {
         if (status.isInBlock) {
           if (import.meta.env.DEV) {
             console.log(`Transaction included in block: ${status.asInBlock}`);