Optimize unknown tokens detection in ModalTokenImport

[raul-oliveira]

Context

Follow-up improvements identified during PR #859 review by @tuliomir.

Improvements

1. Remove tokensBalance dependency from unknown token identification

Reference: comment

Currently, tokensBalance is used as a dependency in the unknownTokens memoized computation, but it is not actually needed to identify whether a token is unknown. Decoupling this would avoid unnecessary recomputations when only balances change.

Current code (src/components/ModalTokenImport.js):

const unknownTokens = useMemo(
  () => walletUtils.fetchUnknownTokens(allTokens, registeredTokens, tokensBalance, hideZeroBalance),
  [allTokens, registeredTokens, tokensBalance, hideZeroBalance]
);

2. Unify unknown tokens fetching to avoid duplicate calls

Reference: comment

Currently, unknownTokens and hasHiddenZeroBalanceTokens each call walletUtils.fetchUnknownTokens separately — once with hideZeroBalance and once without. Instead, we should fetch all unknown tokens once and then filter for zero balance, avoiding the duplicated logic.

Current code:

const unknownTokens = useMemo(
  () => walletUtils.fetchUnknownTokens(allTokens, registeredTokens, tokensBalance, hideZeroBalance),
  [allTokens, registeredTokens, tokensBalance, hideZeroBalance]
);

const hasHiddenZeroBalanceTokens = useMemo(() => {
  if (!hideZeroBalance) return false;
  const allUnknown = walletUtils.fetchUnknownTokens(allTokens, registeredTokens, tokensBalance, false);
  // ...
});

Suggested approach: Compute all unknown tokens once (without zero-balance filtering), then derive both unknownTokens (filtered) and hasHiddenZeroBalanceTokens from that single result.