feat(website): add timetable slot comparison view

[eugenewong22]

Stacked on #4460. This PR is built on top of the save-slots PR and includes its commit, so it should be reviewed and merged after that one. Until then its diff shows both the slots feature and the comparison view.

Context

Completes #4455: with multiple saved timetables (the save-slots PR), users also want to compare arrangements side-by-side instead of using OS split screen or screenshots.

What this does

• Compare… in the slot switcher's ⋯ menu (shown when a semester has ≥2 slots) replaces the timetable with two read-only grids side by side, defaulting to the active slot vs. the next one.
• Each pane has a dropdown to pick any slot, a course count, and a Use this timetable button that switches to that slot and exits compare mode (the active slot's pane shows a disabled "Current timetable" instead).
• Grids render in vertical orientation so they pair naturally; below the md breakpoint the Bootstrap columns stack, which is the mobile behaviour.
• Compare mode is plain component state — no new route. (A ?compare= deep link is a possible future enhancement.) It resets on semester change.

Implementation

• TimetableSlotsCompare renders the bare Timetable grid directly (the VenueDetails pattern) — the heavy interactive TimetableContent isn't involved. Per pane: getSlotTimetableData → hydrateSemTimetableWithLessons (hidden modules omitted) → colour via fillColorMapping with the slot's own saved colours → arrangeLessonsForWeek.
• Slot modules are fetched on mount in case they're missing from the module bank; hydration tolerates missing modules while they load.

Testing

• Component tests with a real store: two grids render from an active + an inactive slot, pane pickers, "Use this timetable" switches state and exits, close button.
• pnpm run ci passes.
• Manually verified in Chromium: enter/exit compare, correct modules in each pane, mobile stacking (screenshot), "Use this timetable" restores the right arrangement.

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[codecov]

Codecov Report

❌ Patch coverage is 91.86992% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.99%. Comparing base (988c6fd) to head (5e6e72a).
⚠️ Report is 264 commits behind head on master.

Files with missing lines	Patch %	Lines
website/src/views/timetable/TimetableContainer.tsx	56.52%	10 Missing ⚠️
...ite/src/views/timetable/TimetableSlotsSwitcher.tsx	86.66%	8 Missing ⚠️
website/src/actions/moduleBank-lru.ts	66.66%	1 Missing ⚠️
website/src/actions/timetables.ts	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4461      +/-   ##
==========================================
+ Coverage   54.52%   59.99%   +5.46%     
==========================================
  Files         274      318      +44     
  Lines        6076     7539    +1463     
  Branches     1455     1858     +403     
==========================================
+ Hits         3313     4523    +1210     
- Misses       2763     3016     +253     

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