Docs: add Windows quick start and .env troubleshooting

[anushkakedari]

Summary

• Add a “Quick start (Windows, local development)” section to README.md.
• Document a simple local setup flow using minimal.env → .env and the existing dev:server / dev:vue scripts.
• Add a “Troubleshooting (common startup issues)” section covering:
• ENCRYPTION_KEYS is a required property, Threat Dragon server cannot start without it
• Unable to find .env file, falling back to environment variables

Motivation

• When setting up the project on Windows, I ran into startup failures because .env and ENCRYPTION_KEYS were not configured.
• These changes make the local development setup clearer and help new contributors quickly fix the most common env-related errors.

Notes

• Documentation-only change: no code or behavior modifications.
• Tested by starting the backend (npm run dev:server) and frontend (npm run dev:vue) with the documented steps and confirming the app loads at http://localhost:8080/.

[jgadsden]

hello @anushkakedari , please follow the 'Contributing to OWASP Threat Dragon' guidelines