refactor: custom numberpad

[ovitrif]

Closes: #309
Closes: #310

This PR refactors the custom numeric keyboard and its associated view amount display.

Description

• replace Keyboard component with new NumberPad written from scratch
• refactor currency types, formatting methods and related components
• remove redundant logic for handling node not running in receive amount screen
• fix(e2e): use single text view for amount + placeholder
• fix(receive): dont request liquidity for 0 LN channels
• fix(receive): disable continue button if cjit amount is below min

Next steps:

• Replace all AmountInput usages with the new NumberPad (ie. replace OS keyboard with custom one)
• Replace NumberPadSimple used to for pin input with NumberPad(type = NumberPadType.SIMPLE)

Preview

Modern - Send	Classic - Transfer
modernSend.mp4	classicTransfer.mp4

QA Notes

• added unit tests for 45+ scenarios in AmountInputViewModelTest

Tests

• numberpad - modern + fiat:
  • enter sats amount of 5 digits
  • switch to fiat > tap a key → no input changes (check both small and large text) ✓
  • delete all → placeholder = 0.00 ✓
  • enter 0., tap delete twice → entire input value deleted ✓
  • when input is empty tap . → input value = 0. ✓
• numberpad - classic:
• try adding more than 8 decimals → should not ✓
• enter 0., tap delete twice → expect entire value deleted
• when input is empty tap . → expect 0. as value
• fix for NumberPad issues in fiat mode #309:
  1. Decimal Point Input Blocking (double-tapping "." blocks input) → unit testes
  2. Decimal Digit Limitation (unlimited digits after decimal in fiat) → unit testes
• fix for NumberPad issues in classic BTC denomination #310:
  1. Classic BTC numberpad shows "000" instead of "." button
  2. Broken Currency conversion calculation when switching units
• regression: numpad in each flow:
  • SendAmountScreen: send > amount
  • ReceiveAmountScreen: on wallet wo. LN >receive > tap receive on LN switch
  • EditInvoiceScreen: receive > edit
  • SpendingAmountScreen: transfer to spending
• regression: edit invoice:
  1. receive > edit > enter amount > show qr → edit → amount persists ✓
  2. receive > edit > enter amount > close sheet → reopen → amount resets ✓
• regression: pin input::
  • settings > security > pin > sanity check pin input