Balances and pricing now shown in credits

Workspace balances, billing pages, transaction history, top-up packs, low-balance hints, auto-refill thresholds, referral banners, studio diagnostics, and admin tools now display amounts as "credits" with smart 2-decimal formatting instead of raw token counts. Pricing and stored values are unchanged — only the on-screen numbers are easier to read (e.g. 105,819 → 105.82 credits).