Add project scaffolding

Makefile, Dockerfile, CI workflow, prettier config, manifests for
Chrome (MV3) and Firefox (MV2), source directory structure, and
minimal test suite. All checks pass.

src/shared/constants.js

@@ -0,0 +1,22 @@
+const ETHEREUM_MAINNET_CHAIN_ID = "0x1";
+
+const DEFAULT_RPC_URL = "https://eth.llamarpc.com";
+
+const BIP44_ETH_PATH = "m/44'/60'/0'/0";
+
+const ERC20_ABI = [
+    "function name() view returns (string)",
+    "function symbol() view returns (string)",
+    "function decimals() view returns (uint8)",
+    "function balanceOf(address) view returns (uint256)",
+    "function transfer(address to, uint256 amount) returns (bool)",
+    "function allowance(address owner, address spender) view returns (uint256)",
+    "function approve(address spender, uint256 amount) returns (bool)",
+];
+
+module.exports = {
+    ETHEREUM_MAINNET_CHAIN_ID,
+    DEFAULT_RPC_URL,
+    BIP44_ETH_PATH,
+    ERC20_ABI,
+};