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.

[vercel]

@eugenewong22 is attempting to deploy a commit to the modsbot's projects Team on Vercel.

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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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[leslieyip02]

@greptile

[greptile-apps]

Greptile Summary

This PR adds multiple saved timetables for each semester. The main changes are:

• Persisted timetable slots with create, duplicate, rename, delete, and switching controls.
• Slot-aware module-cache pinning and loading before a saved slot becomes active.
• A shared-timetable import option that creates a separate Imported slot.

Confidence Score: 4/5

• The import-as-new-timetable undo flow leaves persisted slot state different from the state before import.
• One confirmed user-visible state restoration bug affects the new import workflow.

Files Needing Attention: - website/src/views/timetable/TimetableContainer.tsx

T-Rex Logs

What T-Rex did

• An environment blocker was encountered during setup: ldd reported 15 missing Chromium dependencies, and the Playwright trace showed Chromium exiting with code 127 before a browser page could open, so no UI proof could be captured.
• A focused React/Redux Vitest flow loaded a saved timetable, clicked Import as new timetable, and invoked the Undo handler; the Undo operation reproduced an extra slot (slot 2: Imported) and did not revert to the pre-import state, which had slots 0 and 1 with active slot 1.

_{Ran code and verified through T-Rex}

Prompt To Fix All With AI

### Issue 1
website/src/views/timetable/TimetableContainer.tsx:112-113
**Undo leaves an extra slot**
`addTimetableSlot` is dispatched before the watched `SET_TIMETABLE` import action. Undo therefore restores the state after the slot was created: the imported lessons are removed but the new empty, active `Imported` slot remains. This makes the import notification's `Undo` action fail to restore the prior saved-timetable state; make the combined import atomic for undo, or remove the new slot when its import is undone.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "feat(website): support multiple saved ti..." | Re-trigger Greptile}

[greptile-apps]

Undo leaves an extra slot
addTimetableSlot is dispatched before the watched SET_TIMETABLE import action. Undo therefore restores the state after the slot was created: the imported lessons are removed but the new empty, active Imported slot remains. This makes the import notification's Undo action fail to restore the prior saved-timetable state; make the combined import atomic for undo, or remove the new slot when its import is undone.

Knowledge Base Used:

• Client state and persistence
• Timetable experience

Artifacts

Shared timetable Undo flow test

• Focused React and Redux test that loads a shared timetable, imports it into a new slot, invokes the real notification Undo handler, and compares the resulting slot state with the saved pre-import state — the equality assertion is expected to fail.

Undo retains Imported timetable slot

• Verbose Vitest output for the shared-timetable flow showing the state before import, after import, and after Undo — Undo leaves active Imported slot 2 while clearing its live lessons.

_{Ran code and verified through T-Rex}

Prompt To Fix With AI

This is a comment left during a code review.
Path: website/src/views/timetable/TimetableContainer.tsx
Line: 112-113

Comment:
**Undo leaves an extra slot**
`addTimetableSlot` is dispatched before the watched `SET_TIMETABLE` import action. Undo therefore restores the state after the slot was created: the imported lessons are removed but the new empty, active `Imported` slot remains. This makes the import notification's `Undo` action fail to restore the prior saved-timetable state; make the combined import atomic for undo, or remove the new slot when its import is undone.

**Knowledge Base Used:**
- [Client state and persistence](https://app.greptile.com/nusmods/-/custom-context/knowledge-base/nusmodifications/nusmods/-/docs/client-state-and-persistence.md)
- [Timetable experience](https://app.greptile.com/nusmods/-/custom-context/knowledge-base/nusmodifications/nusmods/-/docs/timetable-experience.md)

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.