feat: add localhost network for e2e against Anvil

- Declare 'localhost' network in hardhat.config.ts pointing to
  http://127.0.0.1:8545 (chainId 31337) so deploy scripts work
  against an external Anvil/Hardhat node.
- Make MNEMONIC and ALCHEMY_API_KEY optional: fall back to the
  Hardhat/Anvil deterministic mnemonic when MNEMONIC is missing,
  and only require ALCHEMY_API_KEY when targeting public networks.
- Reset deploys/localhost.json to a starter state (signers populated,
  contract addresses empty) so the frontend e2e setup writes fresh
  addresses on every run.

deploys/localhost.json

@@ -3,6 +3,6 @@
     "0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266",
     "0x70997970C51812dc3A010C7d01b50e0d17dc79C8"
   ],
-  "p2pix": "0xC86042E9F2977C62Da8c9dDF7F9c40fde4796A29",
-  "token": "0xD38D6367f452D097ccBfDe4490b7de570B6A72Db"
+  "p2pix": "",
+  "token": ""
 }