Add ethers.js and libsodium, document crypto policy

Runtime deps: ethers 6.16.0 (all Ethereum operations) and
libsodium-wrappers-sumo 0.8.2 (Argon2id + XSalsa20-Poly1305 for
encrypting secrets at rest). README now documents all dependencies
with versions/licenses, a crypto policy forbidding raw primitives
in application code, and the updated encryption scheme.

package.json

@@ -18,5 +18,8 @@
         "prettier": "^3.8.1",
         "tailwindcss": "^4.2.1"
     },
-    "dependencies": {}
+    "dependencies": {
+        "ethers": "^6.16.0",
+        "libsodium-wrappers-sumo": "^0.8.2"
+    }
 }