[FEAT] Improve design/UX and implement dark mode as default

[BenGWeeks]

Summary

Improve the overall design and user experience of the marketplace, with dark mode as the default theme.

Current State

The marketplace currently uses a light theme by default. Many users prefer dark mode, especially in the Bitcoin/Lightning community where dark interfaces are common (e.g., most Lightning wallets, Nostr clients).

Proposed Changes

Phase 1: Dark Mode

• Implement dark mode theme
• Make dark mode the default
• Add toggle to switch between light/dark modes
• Persist user preference in localStorage

Phase 2: Design/UX Improvements

• Modernize the overall visual design
• Improve product card layouts
• Better mobile responsiveness
• Cleaner navigation
• Improved loading states and feedback
• Better error messages and empty states

Benefits

• Better alignment with Bitcoin/Lightning ecosystem aesthetics
• Reduced eye strain for users
• More modern, professional appearance
• Improved user experience overall

Technical Notes

The marketplace uses Quasar Framework which has built-in dark mode support via $q.dark.set(true). This should make implementation straightforward.

Perhaps take design inspiration from LNBits.com. Will Diagon Alley inherit the design theme from the LNbits installation?