Use ethers.js Mnemonic for real BIP-39 phrase generation

Replace stub wordlist with ethers.Mnemonic.fromEntropy() using
crypto.getRandomValues(). Add esbuild to bundle popup JS so it
can import ethers directly — no background messaging needed.
Each die click now generates a valid, random BIP-39 mnemonic.

package.json

@@ -14,6 +14,7 @@
     },
     "devDependencies": {
         "@tailwindcss/cli": "^4.2.1",
+        "esbuild": "^0.27.3",
         "jest": "^30.2.0",
         "prettier": "^3.8.1",
         "tailwindcss": "^4.2.1"