feat: register-style fiat input on the zap dialog

[dmnyc]

Summary

Mirrors the iOS PR (barrydeen/wisp-ios#82). Custom-amount input on the zap dialog is fiat-aware. In fiat mode the user types digits register-style — they fill from the cents place, so 21 reads as $0.21 and 2100 reads as $21.00. The hero amount, custom input, and Send button all show the formatted dollar string with the configured currency's symbol. The Send-button label is "Send $X.XX" (was "Zap $X.XX") and the in-progress label on the live-stream zap button is "Sending…" / "Send" instead of "Zapping…" / "Zap" when fiat mode is on. Sub-dollar fiat amounts across the app drop trailing zeros ($0.84 instead of $0.840, $0.8 instead of $0.800).

Implementation notes

• VisualTransformation for the live cursor: the field is bound to the raw digit string ("21") and a CentsVisualTransformation renders the formatted dollar view ($0.21). Compose's cursor lives in raw-string coordinates and the OffsetMapping pegs it to the end of the formatted view, so backspace removes the rightmost digit cleanly. The earlier String-only attempt — bind the field to the formatted string and re-format in onValueChange — broke backspace on Android: when Compose's internal text changed from $0.2 to our re-formatted $0.02, it couldn't map the old cursor position into the new string and snapped it to the start, so subsequent backspaces just moved the cursor instead of deleting.
• ExchangeRateRepository.fiatToSats(majorAmount, currency) — inverse of satsToFiat. Used at the input boundary to convert (cents / 100) dollars → sats via the cached BTC-to-fiat rate.
• AmountFormatter.renderCurrency patterns switch to # (optional) past the decimal for sub-dollar tiers so trailing zeros drop. Whole-dollar amounts still pad to two places ($1.00) matching retail convention.
• zap_x_amount string changes from "Zap %1\$s" to "Send %1\$s" (English only — other locales fall back to English; this string isn't translated yet).

Files

• app/src/main/kotlin/com/wisp/app/repo/ExchangeRateRepository.kt
• app/src/main/kotlin/com/wisp/app/ui/component/ZapDialog.kt
• app/src/main/kotlin/com/wisp/app/ui/screen/LiveStreamScreen.kt
• app/src/main/kotlin/com/wisp/app/ui/util/AmountFormatter.kt
• app/src/main/res/values/strings.xml

Test plan

• Fiat mode on, tap a profile or post → Zap → Custom → type 21 → field reads $0.21, hero matches, Send button reads Send $0.21.
• Continue to type 2100 → field reads $21.00. Backspace → $2.10 → $0.21. Cursor stays at the end; backspace removes the rightmost digit (no cursor-stuck-left-of-symbol bug).
• Open a live stream in fiat mode → header zap button reads Send / Sending… / Sent! (was Zap / Zapping… / Sent!). Toggle fiat mode off — wording reverts.
• Sub-dollar quick-amount chips render as $0.84 (not $0.840); whole-dollar values still render as $1.00.
• Fiat mode off — input is plain integer sats, button reads Zap N sats, no behavioural change.