bdk_demo: add receive address screen

[j-kon]

Summary

• replace the /receive placeholder with a focused receive-address screen
• show the persisted external address as QR and selectable text with its derivation index
• add copy confirmation plus initial, loading, error, retry, and no-wallet states
• cover the page and route with widget tests

This is the UI follow-up to #80 and merged PR #79. It does not change address-generation or persistence behavior.

Closes #83

Testing

• dart format --output=none --set-exit-if-changed lib test
• flutter analyze
• flutter test (155 tests passed)