feat: scaffold transaction presentation in bdk_demo active wallet flow

[j-kon]

Summary

Builds on top of the reference-app scaffold by adding a lightweight transaction presentation section to the active wallet flow in bdk_demo.

Changes include:

• routing activeWallets to a real scaffold page
• placeholder wallet metadata for the reference wallet screen
• placeholder transaction rows showing amount, shortened txid, and confirmed/pending status
• empty transaction state coverage
• README updates clarifying this is scaffold/demo behavior, not real synced wallet functionality

Motivation

This keeps the demo moving incrementally in the scaffold direction from #58 without introducing real wallet sync or transaction fetching yet.

Notes

• no bindings or native code changes
• no real wallet creation, sync, or BDK transaction fetching
• widget tests updated for placeholder transaction and empty-state coverage