feat: improve bdk_demo onboarding and UI states

[j-kon]

Summary

Improves the bdk_demo app to make it a clearer Flutter reference for bdk_dart.

Changes include:

• structured UI with idle/loading/success/error states
• clearer onboarding copy explaining what the demo does
• improved status messaging and feedback
• better presentation of returned wallet/demo data

Motivation

Moves the demo closer to a real reference app, as discussed in #5 and #16.

Notes

• No changes to bindings or native code
• Focused only on bdk_demo/
• Tests updated to reflect new UI behavior

[reez]

@Johnosezele if you'll review

[Johnosezele]

This PR is a UI polish/bindings smoke-test pass, not progress on the Issue #5 reference-app MVP implementation roadmap.
I recommend closing it as out of scope.
It also can’t be cleanly rebased as is now that #58 has merged.

[j-kon]

Thanks, that makes sense.

I see how this is more of a UI polish pass rather than aligned with the reference app roadmap from #58.

I’ll close this and focus on smaller, scoped contributions that build on top of the scaffold going forward.