Implement email functionality and improve booking system

Features:
- Add real email sending with PHP mail() function
- Create HTML email templates for bookings
- Send booking notifications to bands
- Send confirmation emails to customers
- Add email field to bands table and profile
- Enable guest bookings without login
- Improve form validation and UX
- Add migration script for database updates

This fixes the non-working email system and improves the reservation/booking process significantly.

database.sql

@@ -16,6 +16,7 @@ CREATE TABLE IF NOT EXISTS bands (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     user_id INTEGER,
     name TEXT NOT NULL,
+    email TEXT,
     city TEXT,
     genre TEXT,
     price INTEGER DEFAULT 0,