Refactor rates data layer and update dependencies

[mdrlzy]

📌 Description

• Added a forced rates refresh on the splash screen so the Quick screen opens with up to date rates
• Added listeners for rates update flow on screens to refresh data when rates change
• Added throttling for remote rates updated_at checks to avoid unnecessary API checks happening many times per minute
• Added thousands separators for amount input fields
• Replaced Retrofit with Ktor because Ktor is Kotlin-first and better fits future support for multiple API variants, such as premium subscription API
• Updated Kotlin, Gradle, and project dependencies