docs: document local setup, required env vars, and stellar auth flow

[HassanKorey]

Description

Closes #346

This PR standardizes the local setup documentation across the repository and explicitly documents the Stellar web authentication (SEP-10) flow. It ensures new contributors have a single source of truth for bootstrapping the database, configuring their wallet, and successfully authenticating against the local dev environment.

What Changed:

• app/.env.example: Enumerated all environment variables with one-line descriptions, explicit (Mandatory) / (Optional) tags, and included the missing STELLAR_NETWORK and STELLAR_HORIZON_URL variables. Changed defaults to localhost:3000 to be safe for local dev.
• app/README.md: Expanded the setup guide to include generating a Stellar Testnet keypair, installing the Freighter extension, and ensuring it is set to Testnet. Added a dedicated "How Authentication Works" section detailing the SEP-10 challenge -> sign -> verify flow with a caveat regarding the STELLAR_NETWORK configuration.
• README.md (root): Standardized package manager commands to use pnpm (aligning with app/README.md) and added a direct pointer to the Next.js app README for setup and auth flow instructions.

Note on Issue Discrepancy: The issue mentioned that STELLAR_SERVER_SECRET, STELLAR_HOME_DOMAIN, NEXTAUTH_SECRET, and CRON_SECRET were missing from app/.env.example. These were actually already present in the file, so I preserved them while adding the missing network variables and descriptions.

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Checklist

• I have tested my changes locally
• I have updated documentation as needed
• I have run npx prisma generate after schema changes
• I have run npx prisma migrate dev or npx prisma migrate deploy as appropriate

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Post-Merge Steps for Maintainers

If this PR includes changes to the Prisma schema: (N/A for this PR)

1. Run the following command to apply the migration to your database:

   npx prisma migrate deploy

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