[caigee-cmd]

Summary

Extracts the token/quota + DeepSeek equivalent-cost UI from external PR #336, layered onto the existing Usage page from #361 instead of replacing it.

• Stats tab unchanged (by model / account / provider)
• Cost tab: quota, DeepSeek list-price equivalent (peak/off-peak), stacked chart, per-model table, official price catalog (incl. deepseek-flash)
• Backend: internal/accounts/model_cost.go, /api/usage/{pricing,cost}, SummarizeModelCosts with the same creditExpr / request_usage_details fallback as feat: usage page, traffic chart metric switch, stats dim/limit, provider log filter #361
• FE: HeroUI/Tailwind grayscale Studio style; no decorative metric icons; lighter cards; CostChart tooltip aligned with TrafficChart

Architecture notes vs #336

• Does not replace /usage with a parallel page
• Does not include WorkBuddy TTFB or log-retention (see sibling PRs)
• exportWindowHours hardcodes 7d until the retention PR lands; then wire logs.RetentionDays()

Test plan

• go test ./internal/accounts/ ./internal/api/ ./internal/console/ ./internal/store/
• npm run lint / npm run sync (frontend build + embedded static)
• Click Cost tab, switch 24h / 7d / all, verify priced DeepSeek rows and unpriced quota-only models

Supersedes the usage/cost portion of #336. Do not merge #336 as-is.