feat: redesign tickets main view and basket view with improved styles and layout

- Updated SCSS for tickets main view and basket view, enhancing visual design and responsiveness.
- Added new calendar functionality in the admin panel for managing ticket availability.
- Implemented error handling for ticket availability in the basket view.
- Introduced a new calendar availability table in the database for tracking ticket dates.
- Added a new link to the calendar in the logged-in user navigation.
- Improved ticket selection logic to disable unavailable tickets in the main view.
- Created guidelines for code standards in AGENTS.md.

upgrade.php

@@ -29,6 +29,13 @@ $migrations = [
 
   'ticket_prices_add_price_weekend' => "ALTER TABLE ticket_prices ADD COLUMN IF NOT EXISTS price_weekend DECIMAL(10,2) DEFAULT NULL AFTER price",
 
+  'ticket_calendar_availability' => "CREATE TABLE IF NOT EXISTS ticket_calendar_availability (
+    ticket_group VARCHAR(64) NOT NULL,
+    available_date DATE NOT NULL,
+    updated_at DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    PRIMARY KEY (ticket_group, available_date)
+  ) ENGINE=InnoDB DEFAULT CHARSET=utf8",
+
 ];
 
 $ok = 0;