feat: support configurable cost display currency

[Meinianda-L]

Issue for this PR

Closes #32485

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This is needed for users whose model pricing is in a non-USD currency (e.g. CNY/RMB) to see usage costs in their own currency.

The PR is self-contained and depends on no unmerged PR (#39425 was closed as superseded). The first commit is the provider-currency propagation from #39425 (kagura-agent's authorship preserved, fixing #38667); the second adds the display-currency feature (closes #32485).

Adds display.currency and display.exchangeRates config options:

{
  "display": {
    "currency": "CNY",
    "exchangeRates": { "CNY": 7.15 }
  }
}

Costs stay tracked in the provider's source currency. When display.currency is set, amounts are converted at the display boundaries: ACP usage_update (both paths), TUI (sidebar, prompt footer, subagent footer) and the app (context usage, context tab) — all routed through one core Currency utility. When it is unset, or no rate exists for the pair, the original amount and currency are shown — values are never relabeled without conversion. An invalid user rate disables conversion for that currency rather than silently falling back. display.exchangeRates (units per 1 USD) overrides the built-in approximate rate table, which is display-only and not billing-authoritative.

This supersedes the approach in #32487 (auto-closed after going stale, not rejected): since the provider currency is propagated from provider config, no separate global source-currency knob is needed — custom prices configured in RMB are covered by the provider-level currency field.

Docs: new "Display" section in packages/web/src/content/docs/config.mdx. Not changed: CLI stats / run footer, which aggregate costs across models without source-currency information.

How did you verify your code works?

% bun test test/currency.test.ts        # packages/core
 15 pass
 0 fail

% bun test test/acp                     # packages/opencode
 135 pass
 0 fail

% bun typecheck                         # repo root
 30 successful, 30 total

New tests cover conversion (identity, cross-rate, overrides, invalid override disables conversion, unknown/empty source currency) and sendUpdate behavior (source-currency passthrough with no config, USD→CNY conversion, no-relabel fallback). Existing app/tui session tests still pass, oxlint clean on changed files, SDK regenerated with script/generate.ts, config decode verified.

Screenshots / recordings

N/A — formatting goes through Intl.NumberFormat with the resolved currency (e.g. with exchangeRates: { "CNY": 7.2 }, a $1.50 session shows CN¥10.80). Happy to add screenshots if helpful.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[Meinianda-L]

Friendly ping for review: @Hona @Brendonovich (code owners for the touched packages/app files) and @StarpTech (assignee of #32485). The PR is self-contained (includes the provider-currency commit from #39425, authorship preserved), so it can be reviewed and merged independently.

[Meinianda-L]

Update: this PR is now self-contained — rebased onto latest dev with #39425's commit included (authorship preserved), so it no longer waits on that PR. Also applied pre-review polish: invalid user rates now disable conversion instead of silently falling back, renamed rates → exchangeRates and Rates → ExchangeRates for clarity, and added negative tests for unknown/empty source currency.