Render slightly different card UI for classes with clashes

Is your feature request related to a problem? Please describe.

When a user clicks on a card, it shows all the other available slots for that class. However, sometimes there can be quite a bit of mental overhead (at least for me) to parse the timetable and figure out which other slots is available AND doesn't clash with my existing classes

Since Coursereg doesn't allow bidding of multiple classes with overlapping schedules, I think it could make sense to have some kind of UI that helps people quickly figure out clashes in their timetable. For example, this could be useful during tutreg (when lecture slots are mostly already fixed and we can't bid for tut that clashes during lectures anyways)

This is especially so for intro classes with more slots e.g. 2040S

Describe the solution you'd like

For cards, we can render a slightly different UI e.g. striped background, or a small icon in one of the corners indicating that there is a timetable clash

Possibly make this UI toggle-able via settings (like prereq tree directions), or just on the timetable page itself (like vertical mode/show titles)

Start by tracing the card UI and timetable page, then review how existing classes are compared for schedule clashes. Decide whether the indicator belongs on cards or should be toggleable through settings or timetable controls. Done means conflicting classes are visibly distinguishable without disrupting the existing course-selection flow.