docs: clarify Google OAuth environment variable placement for frontend and backend

[Mohit-001-hash]

Description

This PR resolves a significant friction point in the local setup instructions regarding Google OAuth configuration. Previously, the README provided the Client ID variables in a generic bash block without specifying which files they belong to. Copy-pasting this into a terminal would result in lost variables and failed authentication.

I've separated the instructions to clearly show exactly where to place the backend configuration (application.properties) and the frontend configuration (.env), ensuring a smooth local setup experience for new contributors.

Related Issue(s)

Fixes #None (just a documentation fix)

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Refactoring (code improvement without functional change)
• 📝 Documentation update
• 🚨 Breaking change (fix or feature that would cause existing functionality to not work as expected)

Architecture Impact

• 🖥️ Frontend (React / Web Dashboard)
• ☁️ Backend (Spring Boot)
• 💻 Other

Checklist:

• My code follows the Contributing Guidelines of this project.
• I have performed a self-review of my own code.
• I have tested these changes locally and they generate no new warnings or errors.
• I have checked that mobile responsiveness is maintained (if making UI changes).

Screenshots (if applicable)

None

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