fix(frontend): centralize API base configuration

[tokenjunkielabs]

Closes #479

Routes the frontend service layer through the existing shared utils/api.ts Axios client and makes VITE_API_URL the single base-URL configuration point.

• migrates transaction history, forecast, cash-flow forecast, withdrawal, and benefits services off per-file Axios/fetch clients
• removes the obsolete VITE_API_BASE_URL path and service-level localhost fallbacks
• removes duplicated service auth-header construction in favor of the shared request interceptor
• preserves existing endpoint versioning by expressing each route relative to the shared /api base
• keeps withdrawal development fallbacks and transaction-history contract-event degradation behavior intact

GrantFox claimant state for #479 was published under the TokenJunkieLabs payout identity before this sponsor submission.