fix : added email format and password length validation to auth route

[tmdeveloper007]

Summary of What Has Been Done:
Added email format validation using a regex check, minimum password length validation (8 characters) for signup, action parameter validation (must be 'signup' or 'login'), graceful handling of invalid JSON body, and trimming of the display_name field.

Changes Made:

• app/api/auth/route.js: added EMAIL_REGEX constant and format validation
• app/api/auth/route.js: added MIN_PASSWORD_LENGTH constant (8) and length check for signup
• app/api/auth/route.js: added explicit action validation to reject unknown values
• app/api/auth/route.js: added try/catch around req.json() for 400 on invalid JSON

Impact it Made:

• Prevents invalid email formats from reaching Supabase signup
• Enforces minimum password strength for new accounts
• Rejects unknown action values early before captcha verification
• Returns appropriate 400 status codes for bad input