Feat - Add interactive availability calendar to reservation question

[RomainLvr]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !39540
• Here is a brief description of what this PR does

Adds an optional interactive calendar to the reservation question on the end-user form, so users can visually browse existing reservations and pick a free slot instead of only typing start/end dates by hand.

• New "Show availability calendar" toggle in the question editor config
• Calendar displays existing reservations for the selected item
• Selecting a free slot fills the start/end date fields automatically
• The selected slot stays highlighted until form submit, another selection, or item change

Screenshots (if appropriate):

Editor - toggle option

End-user form - calendar with reservations

End-user form - slot selected