feat: smart timetable filters, venue propagation, and render optimizations

[upayanmazumder]

Adds three combinable filters to the timetable view:

• Same Building - all venues share the same building prefix (e.g. SJT)
• Close - room numbers are within 30 of each other
• No Mix - slots are either all morning or all evening, not mixed

Filter logic lives in src/lib/filterUtils.ts so it's shared between the active filter and the match-count preview.

venue is now propagated through the full data pipeline (FacultySelector -> utils -> ViewTimeTable -> CompoundTable) since the filters need it.

Also fixed a few things while I was in here:

• useMemo on timetable/faculty data to avoid recomputing on every render
• Faculty filter now resets when timetable data changes (was silently showing empty results before)
• Pagination uses the filtered list index, not the raw one (was breaking navigation when filters were active)
• Share/Download/Save buttons only show when a timetable is actually selected