* style(ui): use standard disclosure button for provider cost details

Replace custom PlainButton with Components.DisclosureButton primitive for the cost details toggle in ProviderCostSection.qml. This ensures consistent disclosure arrow behavior and accessible properties across disclosure controls.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>

* docs(changelog): describe the cost details disclosure in user terms

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Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>
Co-authored-by: Grok Bot <grok-bot@users.noreply.github.com>