bdk_demo: Add receive address screen with QR and copy action

[j-kon]

Summary

Replace the /receive placeholder in bdk_demo with a focused receive-address screen that demonstrates the receive flow already provided by currentReceiveAddressProvider.

This is a follow-up to #78 and #79 and supports the reference-app goals originally discussed in #5.

Scope

• Add a Receive page for the active wallet.
• Generate the next persisted external receive address through currentReceiveAddressProvider.
• Render the generated address as a QR code.
• Display the address and derivation index.
• Add a copy-address action with user feedback.
• Show clear loading, empty, and error states.
• Replace the existing /receive placeholder route.
• Add routing and widget tests for the main states and interactions.

Out of scope

• BIP21 amount, label, or message fields.
• Address sharing.
• Send or broadcast changes.
• Changes to address-generation persistence logic.
• Production-wallet security or payment-request features.

Acceptance criteria

• Opening Receive with an active wallet allows a receive address to be generated.
• The generated address is displayed as text and as a QR code.
• The derivation index is visible.
• The address can be copied with clear confirmation.
• Loading and provider errors are represented in the UI.
• The route no longer displays PlaceholderPage.
• Widget and routing tests cover the receive flow.
• flutter analyze and flutter test pass for bdk_demo.

[j-kon]

This is the UI follow-up to #80 and merged PR #79. #80 covered the provider/state layer; this issue is intentionally limited to the /receive page, QR rendering, copy action, UI states, routing, and widget tests.