fix: improve .env.template with descriptive comments and Base Sepolia RPC example

[schoolkamsergj]

Summary

Improves packages/playground/.env.template to reduce setup friction for developers, as requested in #2644.

Changes

Descriptive comments for every variable

Each env var now explains:

• what it is used for (with a reference to the exact code location)
• whether it is required or optional
• where to get the value, with direct links

New NEXT_PUBLIC_BASE_SEPOLIA_RPC_URL entry

Adds the missing Base Sepolia RPC variable for local development and testing, with a note about the public fallback endpoint (https://sepolia.base.org) for developers who don't yet have a dedicated RPC key.

Source links verified from the codebase

• NEXT_PUBLIC_OCK_API_KEY → used in AppProvider.tsx as OnchainKitProvider apiKey
• NEXT_PUBLIC_PROJECT_ID → used in AppProvider.tsx as OnchainKitProvider projectId
• NEXT_PUBLIC_ETHEREUM_MAINNET_RPC_URL → used in AppProvider.tsx to create a viem public client for mainnet
• NEXT_PUBLIC_VERCEL_ENV → set automatically by Vercel on deploy
• NEXT_PUBLIC_RESERVOIR_API_KEY → used for NFT data features

Before / After

Before:

NEXT_PUBLIC_ETHEREUM_MAINNET_RPC_URL=""
NEXT_PUBLIC_OCK_API_KEY=""
NEXT_PUBLIC_VERCEL_ENV=""
NEXT_PUBLIC_PROJECT_ID=""
NEXT_PUBLIC_RESERVOIR_API_KEY=""

After: each variable has a section header, explanation, required/optional label, and direct links to where to get the value.

Fixes #2644

[vercel]

@schoolkamsergj is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

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