fix: unified invoice validation with zero on-chain balance

[jvsena42]

This PR fixes unified invoice validation to accept Lightning payment when on-chain balance is zero.

Description

When typing or pasting a unified invoice (BIP21 with embedded Lightning), the validation incorrectly rejected it if on-chain balance was zero, even when the user had sufficient Lightning balance. The fix adds unified invoice detection to validateOnChainAddress(), mirroring the existing logic in onScanOnchain().

Changes:

1. Extracts a reusable extractViableLightningInvoice() helper that checks if a BIP21 URI contains a valid, non-expired Lightning invoice with sufficient sending capacity
2. Updates validateOnChainAddress() to check for Lightning payment viability before falling back to on-chain balance validation
3. Refactors onScanOnchain() to use the same helper, eliminating code duplication
4. Fixes on-chain balance checks to only trigger when payment method is on-chain

Preview

zero-amount-invoice.webm

with-amount.webm

QA Notes

1. Unified invoice with Lightning balance only

1. Create wallet with 0 on-chain balance but funded Lightning channel
2. Paste a unified invoice (BIP21 with ?lightning= param)
3. Verify: Address should be accepted (green checkmark)
4. Before fix: Shows "Insufficient Savings" error

2. Regression - Pure on-chain address

1. Paste a plain Bitcoin address
2. Verify on-chain balance validation still works as expected

3. Regression - Scan flow

1. Scan a unified QR code
2. Verify it still routes correctly to Lightning payment

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.