Implement comprehensive dark mode functionality

Features implemented:

• Shop-level default theme setting (database migration included)
• User-level theme preference override
• Theme hierarchy: localStorage > user preference > shop default > system default
• CSS custom properties for comprehensive dark/light theme switching
• Theme toggle button in navigation with real-time switching
• Shop settings UI for default theme configuration
• User settings UI for personal theme preference

Technical implementation:

• Added default_theme column to Shop model
• Enhanced User model theme_id functionality
• CSS variables system for seamless theme switching
• JavaScript localStorage management for persistent user choices
• Template logic respecting theme priority hierarchy

All static inline styles cleanup completed as prerequisite work.

Summary by CodeRabbit

• New Features

  • Light/Dark theme support with automatic selection (user preference, shop default, or saved choice) and instant switching.
  • Theme preference controls in User Settings; default theme selector in Shop Settings.
  • Theme toggle UI.

• UI

  • Comprehensive dark-mode styling across buttons, forms, cards, alerts, and carts.
  • Improved checkout for pending crypto payments with per-quote View and Cancel actions and clearer messaging.
  • Refined empty cart layout and minor button class cleanups.
  • Simplified refund address forms (removed optional labels).

• Documentation

  • Overhauled migration guide with autogeneration flow and backup reminders.
  • Added CSS theme audit report and recommendations.