fix: pass offerings to slotUsedWeight in semester-row so FY units aren't double-counted"

[coldfinity]

Problem

Full-year (FY) units like FIT3170 (12cp, runs across S1+S2) appear in both S1 and S2 of a year, with each twin contributing half the unit's workload per semester. The planner displays usedWeight / capacity units on the left rail to show how full each semester is.

But semester-row was double-counting FY units:

Actual	Before (broken)	After (fixed)
1 × FY 12cp in S1	2 / 4 units	1 / 4 units
1 × FY 12cp in S1 + S2	2 / 4 each	1 / 4 each
2 × FY 12cp + 1 × 6cp in S1	5 / 4 units (overflow!)	3 / 4 units

A student adding a single FY unit would see "2 / 4 units" and think the semester was half-full when it wasn't. With multiple FY units the display could even show usedWeight > capacity.

Root cause

slotUsedWeight in types.ts has an optional offerings parameter. When present, it checks whether a unit is full-year via isFullYearTwinSlot and halves its CP contribution per semester (so a 12cp FY unit = weight 1 per semester, not 2). Without offerings, the FY check is always skipped.

semester-row.tsx was the only call site (out of 10) that omitted offerings:

File	Passes offerings?
semester-row.tsx	❌ → ✅ fixed
semester-slot.tsx	✅
draggable-unit-row.tsx	✅
plan-grid.tsx (6 calls)	✅
progress.ts	✅
validation.ts	✅

Fix

2-line change in packages/webapp/components/planner/semester-row.tsx:

-  const { dispatch, units } = usePlanner()
-  const usedWeight = slotUsedWeight(slot, units)
+  const { dispatch, units, offerings } = usePlanner()
+  const usedWeight = slotUsedWeight(slot, units, offerings)

Testing

pnpm --filter webapp build

To verify

1. Create a plan with a course that includes a full-year unit (e.g. FIT3170 any unit whose offerings are exclusively FULL_YEAR)
2. Add it to planner, it should appear in both S1 and S2
3. Check the left rail label, it should show 1 / 4 units for each semester, not 2 / 4
4. Add another FY unit, should show 2 / 4, not 4 / 4 and overflowing
5. Verify the capacity controls (Add/Remove unit slots) still work correctly
6. Verify normal (non-FY) units are unaffected

Fixes #13.