feat(website): support multiple saved timetables per semester

[eugenewong22]

Context

Closes the "save slot" half of #4455: users want to keep multiple timetable arrangements per semester and switch between them without screenshotting before every change. (A follow-up PR adds the side-by-side comparison view — see the issue for the full plan.)

What this does

• Save slots per semester: a pill-tab row under the semester switcher shows each saved arrangement. Switching tabs swaps the entire arrangement — lessons, colours, hidden modules and TA modules — instantly. Edits always go to the active slot.
• Create / duplicate: a + menu adds a new blank timetable or duplicates the current one.
• Rename / delete: a ⋯ menu with modal dialogs (following ResetTimetable's pattern). Deleting is undoable via the existing undo notification; the last remaining slot can't be deleted.
• Import as new timetable: shared timetable links now offer importing into a fresh slot, so importing no longer has to overwrite the saved timetable. The existing overwrite Import is unchanged.

Implementation

• Swap-on-switch: the active timetable stays in the existing TimetablesState.{lessons,colors,hidden,ta} maps, so the ~15 existing timetable actions, all selectors, undo, share and export code are untouched. A new slots: { [semester]: TimetableSlot[] } array stores arrangements; SWITCH_TIMETABLE_SLOT snapshots the live maps into the outgoing slot and loads the target's data (reducers/timetables.ts).
• Slot data is authoritative for inactive slots only — the active slot's data lives in the live maps and its snapshot is refreshed on switch-away. All reads go through the new getSlotTimetableData selector.
• Lazy initialization + v3 persist migration: semesters with no slots entry implicitly have one active slot ("Timetable 1"). The redux-persist migration (migrateV2toV3, following the planner's exported-migration pattern) only adds two empty maps — no user data is rewritten.
• Undo: DELETE_TIMETABLE_SLOT joins actionsToWatch. Switching/adding is deliberately not watched — switching back is lossless, and with limit: 1 it would clobber the single undo step.
• Module bank: slot modules are pinned against LRU eviction (getPinnedLessonConfigs), and the switch/delete thunks fetch the incoming slot's modules before loading it into the live timetable.
• Year rollover: unchanged — only the active lessons are archived; slots are reset with the rest of the year's state (documented in the reconciler; archiving all slots would need an archive shape change and is left as a follow-up).

Testing

• Reducer: slot CRUD, switch round-trip preserves both timetables exactly, delete-active activates the neighbour, no-ops return the same state reference, migration unit tests (reducers/timetables.test.ts).
• Selectors, thunk ordering (fetch before switch), LRU pinning, undo restore, switcher component behaviour, and an integration test that import-as-new-slot doesn't clobber the saved timetable.
• pnpm run ci passes (lint, typecheck, 800+ tests, build).
• Manually verified in Chromium: create/rename/duplicate/delete/switch, share-import both paths, reload persistence, v2→v3 migration with real persisted v2 state, and cross-tab sync with two tabs.

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[codecov]

Codecov Report

❌ Patch coverage is 94.27083% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.79%. Comparing base (988c6fd) to head (cca1f2c).
⚠️ Report is 264 commits behind head on master.

Files with missing lines	Patch %	Lines
...ite/src/views/timetable/TimetableSlotsSwitcher.tsx	85.45%	8 Missing ⚠️
website/src/actions/moduleBank-lru.ts	66.66%	1 Missing ⚠️
website/src/actions/timetables.ts	96.77%	1 Missing ⚠️
website/src/views/timetable/TimetableContainer.tsx	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4460      +/-   ##
==========================================
+ Coverage   54.52%   59.79%   +5.26%     
==========================================
  Files         274      317      +43     
  Lines        6076     7486    +1410     
  Branches     1455     1846     +391     
==========================================
+ Hits         3313     4476    +1163     
- Misses       2763     3010     +247     

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