Add configurable available-hours layout for the booking flow

[dyanakiev]

This change adds a new booking setting that lets admins choose how available time slots are displayed to customers: as a traditional single-column list or as a more compact grid.

The setting is stored in the database through a migration, exposed in the booking settings page, validated on save, and passed to the frontend booking flow where the available-hours picker now adapts its layout accordingly.

Why this is needed
The current single-column layout works, but it is not always the best fit for every booking flow. Some setups benefit from a denser presentation of time slots, especially when there are many options on the same day. This update gives site owners control over the booking experience without changing the underlying availability logic.

Example scenarios

• A clinic with many short appointments can switch to the grid layout to make time slots easier to scan.
• A business with fewer, longer appointments can keep the list layout for a simpler, more familiar flow.
• Mobile users on screens with limited vertical space can benefit from the compact grid presentation.

Migration Warning

This branch adds a new migration.

My previous PRs also add migrations, so if multiple PRs are merged, please check and renumber the migration files as needed to avoid duplicate migration numbers.