Add fiat-denominated send amounts to the iMessage app

[edwardbickerton]

Closes #67

The iMessage send flow was sat-only. This brings it in line with the main
app so users can enter an amount in their preferred fiat currency and have
it converted to sats.

Changes

• Replace the sat-only TextField with NumericalInputView, the same
  amount-entry component the main app uses (fiat entry + live sat conversion).
• Share the preferred currency between the app and the iMessage extension via
  the existing group.com.cypherbase.macadamia app group (both targets already
  declare the entitlement), so the setting stays in sync.
• Lift the exchange-rate fetch/decode out of AppState into
  Currency.fetchBitcoinExchangeRates() and reuse it from both targets
  (no duplicated logic).
• Migrate any existing PreferredCurrencyConversionUnit value from standard
  defaults into the shared store on launch, so current users keep their setting.
• Only fetch exchange rates in the extension when the preferred unit is fiat.

Testing

Happy to adjust based on review. Worth a build of both the app and the
macadamiaMessages extension and a quick run through the iMessage send flow
with a fiat preference set to confirm conversion and token creation.

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• I made sure to choose branch RELEASE as the target for this pull request