feat: add Supabase login authentication

[neethika12]

Closes Sentinent-AI/Sentinent#18.

Changes

• replace the mock frontend auth service with Supabase email/password authentication
• add Supabase client setup and environment configuration placeholders
• update login, signup, password reset, invitation acceptance, and route guarding for async session checks
• extend Angular unit tests and README setup instructions for the new auth flow

Testing

• npm test -- --watch=false --browsers=ChromeHeadless
• build was attempted locally, but the sandbox did not return a usable error/output for the final build step