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.

tests/constants.test.js

@@ -0,0 +1,29 @@
+const {
+    ETHEREUM_MAINNET_CHAIN_ID,
+    DEFAULT_RPC_URL,
+    BIP44_ETH_PATH,
+    ERC20_ABI,
+} = require("../src/shared/constants");
+
+describe("constants", () => {
+    test("exports expected chain ID", () => {
+        expect(ETHEREUM_MAINNET_CHAIN_ID).toBe("0x1");
+    });
+
+    test("exports a default RPC URL", () => {
+        expect(typeof DEFAULT_RPC_URL).toBe("string");
+        expect(DEFAULT_RPC_URL.startsWith("https://")).toBe(true);
+    });
+
+    test("exports BIP-44 Ethereum derivation path", () => {
+        expect(BIP44_ETH_PATH).toBe("m/44'/60'/0'/0");
+    });
+
+    test("exports ERC-20 ABI with expected functions", () => {
+        expect(Array.isArray(ERC20_ABI)).toBe(true);
+        expect(ERC20_ABI.length).toBeGreaterThan(0);
+        const joined = ERC20_ABI.join(" ");
+        expect(joined).toContain("balanceOf");
+        expect(joined).toContain("transfer");
+    });
+});