Always render the field now (label included) with just its value
empty when unknown, rather than hiding the whole field.
In Manual source mode it's now a genuine input: typing into it
recomputes the "BTC to currency" rate the same way typing into that
field recomputes this one. Both edit the same underlying
ManualExchangeRateSource.rate via the same self-inverse division
(CurrencyConverter.satsPerCurrencyUnit of a rate gives Sats, and of a
Sats amount gives a rate back), tracked as a new manualSatsPerCurrencyInput
field alongside the existing manualRateInput.
Wired through PriceViewModel, the iOS bridge/ConverterViewModel, and
both UIs.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy