chore: add .env.example and update setup instructions

Author: MuzzaiyyanHussain

.env.example

@@ -0,0 +1,18 @@
+# ===============================
+# OnchainKit Environment Example
+# ===============================
+
+# Coinbase OnchainKit API Key
+# Replace with your actual key (if you have one)
+# You can leave this empty for open-source / Storybook builds
+NEXT_PUBLIC_ONCHAINKIT_API_KEY=
+
+# Wagmi / Ethereum Chain Settings
+# Example: base, mainnet, polygon, etc.
+NEXT_PUBLIC_DEFAULT_CHAIN=base
+
+# Analytics or telemetry flags (optional)
+ENABLE_ANALYTICS=false
+
+# Storybook environment flag
+STORYBOOK_ENV=local