Fix: use swapExactETHForTokens instead of swapETHForExactTokens

[draevik]

What this fixes

The native-in swap branch in swap() (hooks/useWeb3Functions.ts) called swapETHForExactTokens — an exact-output router function — while passing it amountOutMin, a slippage floor, not an exact target. The call still succeeds (the user's input amount is normally enough to cover the floor amount), so it never reverts — it just silently pins the received tokens to the worst-case slippage amount every time, refunding the unused ETH, instead of delivering the real market-rate output.

Before (hooks/useWeb3Functions.ts):

if (fromAddress === zeroAddress) {
  path = [weth, toAddress];
  methods = [
    "swapETHForExactTokens",
    "swapExactETHForTokensSupportingFeeOnTransferTokens",
  ];
}

After (matches config.governor-style consistency — i.e. matches the swapExact... pattern already used correctly by the other two trade directions a few lines below):

if (fromAddress === zeroAddress) {
  path = [weth, toAddress];
}
// ...
const methods = getSwapMethods(
  fromAddress === zeroAddress,
  toAddress === zeroAddress
);

What this adds

Extracted the trade-type → router-function mapping into a small, dependency-free helper, lib/swap-method.ts, so this logic is unit-testable without a wallet or RPC connection. Repo previously had zero test infrastructure (package.json had no test script), so this also adds vitest as a devDependency and a test script.

lib/swap-method.test.ts covers all three trade directions and explicitly asserts the native-in case returns swapExactETHForTokens, not swapETHForExactTokens.

Verification

• Reverted the helper to the original buggy string locally and confirmed pnpm test fails (2 of 4 assertions), then restored the fix and confirmed all 4 pass — the test genuinely catches this regression, not just a green checkmark by coincidence.
• Ran pnpm test on the final diff: 4/4 passing.
• Ran pnpm lint and pnpm build: both clean (aside from a pre-existing, unrelated @metamask/sdk React Native resolution warning present on main too).
• Confirmed the fix doesn't touch the token→ETH or token→token branches, which already used the correct swapExact... pattern.
• Manual on-chain verification on testnet was attempted but blocked — see note below.

Note on manual testnet verification

Wanted to confirm real quoted-vs-received amounts on-chain before/after this fix, but hit a blocker unrelated to this change: the app's testnet network isn't currently selectable in the UI at all (config/index.ts only lists lcai in chains, so config/wagmi.ts never offers testnet as a connectable network). Separately, the router address the explorer associates with real swap activity on testnet doesn't match what's committed to config/index.ts for lcaiTestnet — worth a maintainer sanity check, but it's a config/deployment question, not something this PR should guess at. Flagging both as follow-ups rather than blocking this fix on them, since neither affects mainnet or the correctness of the change itself.

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