Styling hooks

Root class: `.rocwp-wcphoto-booking` plus theme modifier `.rocwp-wcphoto-theme-light`/`dark`.

Calendar classes: `.rocwp-calendly__*`; slots: `.rocwp-wcphoto-slot` with `.is-unavailable` and `.is-selected`.

Confirmation bar appears when `[data-rocwp-confirm]` is present; otherwise the inline button sits next to the selected slot.