[Enhancement] Add banner image, introduction, carousel, and help section to extension UI

[BenGWeeks]

Problem

The nostrclient extension UI lacks visual polish and helpful onboarding content. Admin users see the relay management interface without context about what nostrclient does or how to use it.

Suggested Improvements

Looking at the Allowance and Boltz extensions as good examples:

1. Banner Image

• Add a small banner at the top of the right column (like Allowance/Boltz)
• Nostr + Bitcoin branding
• Makes the extension more visually professional

2. Introduction Section

Add a brief introduction explaining:

• What nostrclient is (relay multiplexer)
• Why use it (simplified configuration, always-on, resource efficient)
• Key benefit: Connect once, reach multiple relays

3. Carousel Sections (like Allowance)

Create informative carousel panels:

• Quick Start: Adding relays, configuring endpoints
• Usage Guide: Connecting Nostr clients to your endpoint
• About Nostr: Brief protocol explanation and why multiple relays matter

4. Help Section

Add a help section with:

• Feedback/Suggestions: Link to raise issues on GitHub
• GitHub Repository: Link to https://github.com/lnbits/nostrclient
• Common Questions: Basic troubleshooting