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.

.serena/project.yml

@@ -115,3 +115,10 @@ initial_prompt: ""
 # override of the corresponding setting in serena_config.yml, see the documentation there.
 # If null or missing, the value from the global config is used.
 symbol_info_budget:
+
+# The language backend to use for this project.
+# If not set, the global setting from serena_config.yml is used.
+# Valid values: LSP, JetBrains
+# Note: the backend is fixed at startup. If a project with a different backend
+# is activated post-init, an error will be returned.
+language_backend: