bug: amount keypad floats mid-sheet instead of sitting above continue #1056

What happened?

On Bitkit iOS 3.0.1 (202), the send flow's Amount step has the keypad floating in the middle of the sheet with a large gap above it, and extra side padding. Same issue on Android: synonymdev/bitkit-android#1627.

USDT Amount step (balance masked):

Expected behavior

Intended Amount-screen layout, top to bottom:

1. A flexible space at the top, with the amount stack (the secondary ₿ or fiat line and the large amount) vertically centred in it.
2. The controls bar (Available, the asset chip, Max, the network chip) right above the keypad, with no large gap before it.
3. The keypad at full width within its bounds, with no extra side padding, so the edge keys reach the container edges.
4. Continue at the bottom.

Today the keypad floats in the middle with a large gap above it and extra side padding. Compare against the Figma design for the Amount step if one exists.