fix: cap send amount pad at available balance

[CypherPoet]

Fixes synonymdev#346

Description

This PR makes the send amount number pad refuse entry above your available balance. Before, you could type any amount and the screen only disabled the Continue button once you went over; now the number pad itself stops you at the maximum you can send, using the same haptic and error flash it already shows at its upper input limit.

The cap follows the selected wallet: spendable on-chain balance minus the network fee for savings, or outbound capacity minus the routing-fee estimate for spending. Tapping the Available label still fills the exact maximum, and the Continue-button validation stays in place as a backstop.

Linked Issues/Tasks

Fixes synonymdev#346

Screenshot / Video

346-onchain-send-cap.mp4

QA Notes

Manual Tests

• 1. Send -> paste/scan on-chain invoice -> Amount: type past the displayed Available: amount freezes at the max, haptic + error flash, can't exceed.
  • 1b. tap the Available label: fills exactly the max, Continue enabled.
  • 1c. regression: enter a normal under-balance amount: Continue enabled, proceeds to Confirm.
• 2. Spending selected -> Amount: type past Available: capped at outbound max minus the routing-fee estimate.
• 3. regression: zero available balance -> Amount: pad still accepts input (not frozen), Continue stays disabled.

Automated Checks

• Unit tests added: dynamic input cap in BitkitTests/NumberPadTests.swift (testMaxAmountOverrideBlocksInputAboveBalance, testClearingMaxAmountOverrideRestoresGlobalCap).
• Built for iOS Simulator (Debug) locally; BitkitTests/NumberPadTests passes (24 tests).
• Verified on-device (iOS 26.4 simulator, regtest): on-chain Send number pad froze at the available balance and rejected over-max input (see recording).
• CI: standard build/test checks run by the PR bot.

[CypherPoet]

Superseded by #4: the head branch was renamed from claude/inspiring-proskuriakova-e5b6ac to fix/send-amount-cap to drop the auto-generated prefix. Closing in favor of #4.