On mobile and Capacitor native, replace the OS decimal keyboard
for locale-aware amount fields with a Cashtab-styled numeric keypad
(digits, locale decimal separator, backspace; no math operators). This
keeps locale formatters applied consistently.
For now we implement only on the send screen. Each callsite has some UX considerations. The point of using a custom keypad is so the user can do all their actions in the same view without having to dismiss they keypad e.g. to push "Send." Additional benefit of standardizing expected decimal and thousands separators to the users actual locale.
We disable the "Advanced" features on the Send screen in mobile (we hide the input). Mobile users have these features if they copy paste or scan a bip21 QR code. but imo mobile users are not typing in csvs or OP_RETURN_RAW so it's better UX to hide them.
Will extend across Send, Receive, Token, Airdrop, Agora Slider, and related inputs in later diffs. Each callsite has unique UX considerations.
