[Feature]: Display usage costs in a selected local currency

[XDGFX]

Before submitting

• I searched existing discussions. I did not find a duplicate.
• This is a concrete use case, not a vague idea.

Area

Usage reporting: packages/shared, web usage page, and mobile usage screen.

Problem or use case

The Usage page shows API-equivalent token cost only in USD. This makes the figure harder to relate to a personal budget or local invoice for users outside the US.

The underlying value should remain USD: T3 prices transcript usage against USD model rates, and the result is already an estimate rather than a subscription charge. The request is only to make that estimate readable in the users chosen currency.

Proposed solution

Add a Usage display-currency preference:

• USD by default.
• Either detect the device locale or allow an explicit ISO 4217 currency selection.
• Keep price calculation, aggregation, and stored values in USD. Convert only when rendering.
• Apply the selected currency consistently to totals, provider and model tables, cache savings, charts, axes, and tooltips on web and mobile.
• Show the rate, source, and update time, for example: Estimated API-equivalent cost · AUD; 1 USD = 1.54 AUD, updated 19 Aug 2026.
• Cache the most recent rate so usage remains viewable offline, and label stale data clearly.

Why this matters

The existing USD estimate is useful, but local-currency display makes it immediately useful for people who budget and pay in another currency. Keeping USD canonical preserves existing calculations and avoids suggesting that the converted total is an exact provider bill.

Smallest useful scope

A display-only currency setting for the Usage page, initially supporting USD and the users selected currency, with a cached USD exchange rate and a visible rate timestamp.

Alternatives considered

• Keep USD only: simple and unambiguous, but less useful outside the US.
• Change stored cost values to local currency: this would make historical data dependent on exchange-rate timing and is unnecessary.
• Show a currency symbol without conversion: misleading.

Risks or tradeoffs

Exchange rates add a small data dependency and a freshness/offline policy. Rates should be explicitly labelled as display conversion only; T3 should not imply that the local value is the actual amount billed by a subscription provider.

Contribution

• I would be open to helping implement this.