fix(frontend): getUpcomingPurchases declares an array but the API returns { purchases } #1189

Summary

getUpcomingPurchases() in the frontend API layer declares Promise<UpcomingPurchase[]>, but /api/dashboard/upcoming returns an object with a purchases array. The dashboard consumers cast around the mismatch instead of using the real type.

Evidence (origin/main)

• frontend/src/api/*: getUpcomingPurchases(): Promise<UpcomingPurchase[]> calls apiRequest<UpcomingPurchase[]>('/dashboard/upcoming').
• Tests already mock the real shape (dashboard-ownership-950.test.ts:159 resolves { purchases: [...] }).
• The Go handler returns { purchases: [...] } (internal/api/handler_dashboard.go, getUpcomingPurchases).

Proposed fix

Declare Promise<{ purchases: UpcomingPurchase[] }>, drop the casts in the dashboard consumers, and let tsc check both call sites.

Found by CodeRabbit on #1186. The mismatch predates that PR.