Add shortcode for check-in day of the week

Problem Statement:

• There is currently a shortcode %check_out_day_of_week% that displays the check-out day (e.g., "Monday")
• No equivalent exists for check-in, making it difficult to include the weekday in guest communications

Current Workaround:

• Using %check_in% which shows the full date (e.g., "Sunday, August 2")
• Manually calculating or noting the weekday separately

Impact:

• Inconsistent messaging — check-out messages can include the weekday, but check-in messages cannot
• Extra manual effort to determine and communicate the check-in weekday to guests

Proposed Solution:

• Add a %check_in_day_of_week% shortcode that displays the check-in day of the week (e.g., "Monday")

Unlock:

• Consistent, automated guest messaging with weekday information for both check-in and check-out
• Improved guest experience without manual intervention