feat: currentCurrency added to AssetsController state

[bergarces]

Explanation

The state for currentCurrency is currently saved in CurrencyRateController. Since that controller is being deprecated, and we need that piece of state to be stored somewhere, it's being moved to the controller that will replace it.

This PR also ensures that PriceDataSource gets updated with the existing currency instead of just being set up at constructor level.

References

• Fixes https://consensyssoftware.atlassian.net/browse/ASSETS-2724

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• [] I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Medium Risk
Introduces new persisted state and changes the PriceDataSourceOptions API, which may require consumer updates and could affect price-refresh behavior if miswired.

Overview
Adds a persisted selectedCurrency (ISO 4217, default usd) to AssetsController plus setSelectedCurrency, which updates state and triggers a one-off getAssets refresh for price data.

Refactors PriceDataSource to take a required getSelectedCurrency() callback (replacing the static currency option) so price fetches always use the latest currency, and updates unit tests/changelog accordingly.

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

[bergarces]

Currency is already included as part of the query key for cache purposes, so no changes needed in the client.