fix: match spent Bitcoin outputs by outpoint

[tantshirt]

Issue Number (if Applicable): Fixes #3176

Description

Bitcoin post-send cleanup removed unspents by transaction hash alone. When a transaction contained multiple wallet-owned outputs, spending one output could also remove its still-unspent sibling from cached transaction history and the in-memory unspent list.

This changes both cleanup paths to identify spent outputs by the complete outpoint: transaction hash and output index. A focused cw_bitcoin unit test covers exact matches, sibling outputs from the same transaction, and matching output indexes from different transactions.

Validation

• flutter test --no-pub test/bitcoin_unspent_test.dart
• flutter test --no-pub (12 tests)
• flutter analyze --no-pub test/bitcoin_unspent_test.dart

Pull Request - Checklist

• Initial Manual Tests Passed
• Double check modified code and verify it with the feature/task requirements
• Format code
• Look for code duplication
• Clear naming for variables and methods
• Manual tests in accessibility mode (TalkBack on Android) passed