fix: useScheduleData returns hardcoded loading/error fields that never vary

[chiptus]

Problem

src/hooks/useScheduleData.ts:60-61, 175-180:

const loading = false;
const error = null;
...
return { scheduleDays, allStages, loading, error };

loading/error are hardcoded constants, never varying — but callers (Timeline.tsx, ListSchedule.tsx) still branch on if (loading) / if (error), code that can never execute. The hook's interface advertises async-state handling it doesn't actually do (the real async state lives in the suspense queries upstream of it).

The grouping/enrichment logic in the hook's useMemo (lines 63-163) is real and worth keeping as-is — this ticket is scoped to the dead loading/error fields only.

Note: distinct from #156, which covers the leftover console.log("no stageId", set) at line 121 in this same file — that's already ticketed; don't duplicate it here.

Fix

Drop loading/error from the hook's return type and delete the dead branches in both callers.

Architecture note

Filed from an architecture review (module/interface/depth vocabulary, see /codebase-design). Interface should shrink to what the module actually does.