tui: show estimated API cost for Anthropic subscription in info widget

The usage info widget only displayed a dollar figure for per-token-billed
providers (OpenRouter, direct API keys). Claude subscription / OAuth users
saw only the 5-hour/weekly limit bars and no cost, unlike OpenRouter.

Now accrue an estimated equivalent API cost for Anthropic OAuth sessions
using real per-model pricing (input/output/cache-read from
anthropic_api_pricing) and render a '~$X.XX est. API cost' line beneath
the usage bars (expanded and compact). Per-token-billed cost behavior is
unchanged; cache-read tokens are now priced at the cheaper cache rate.