feat: load real transaction history in demo app

[j-kon]

Summary

• replace transaction history sample rows with active-wallet-backed data
• map BDK wallet transactions into demo-app history rows
• add a clear no-active-wallet state before loading transaction history
• keep widget tests lightweight by overriding wallet availability instead of creating real BDK wallets
• keep test-only fakes under the transaction feature test helpers

Context

Continues the transaction presentation scaffold from #62 by wiring the list/detail flow to active wallet transaction data instead of sample rows.

The production transaction repository still uses the BDK Dart API to read wallet transactions, while widget tests use fake repositories and a lightweight wallet-availability provider.

Verification

• dart format --output=none --set-exit-if-changed lib test example bdk_demo/lib bdk_demo/test
• dart analyze --fatal-infos --fatal-warnings lib test example
• dart test
• flutter analyze inside bdk_demo
• flutter test --no-pub test inside bdk_demo

Validation passed:

• root Dart tests: 22/22 passed
• bdk_demo Flutter tests: 181 tests passed
• no analyzer issues

Also confirmed:

• transactions_list_page_test.dart does not import package:bdk_dart/bdk.dart
• no widget test creates a real BDK Wallet
• production transactions_repository.dart still imports and uses BDK