Improve demo page with date-based filtering and trip grouping

[chtitux]

Major improvements to the interactive demo:

Location and operator:

• Fix location from Martinique to La Réunion
• Add operator information: Transdev Outre-Mer and GIE Activ'

Date-based filtering:

• Replace calendar list with date picker (defaults to today)
• Show active calendars for selected date
• Filter trips by selected date using new flexible API

Trip organization:

• Group trips by headsign and direction
• Order groups by direction_id, then by headsign
• Show trip count for each group
• Improved visual hierarchy with grouped layout

UI improvements:

• Modern date input with focus styles
• Better trip grouping with headers
• Cleaner information display
• Active calendar info instead of full calendar grid

All 77 tests pass.