Add device debug info box

[iAndrea]

With this implementation we can keep an eye on the refresh date (and more) for every plugin without having to access the logs.

Added

• Global Debug Mode: Device-level toggle in settings UI to enable/disable debug overlay across all plugins
• Plugin-Level Override: Per-plugin debug control via displayRefreshTime setting
• Debug Overlay Display: White box in top-right corner showing:
  • Last refresh timestamp (timezone/format aware)
  • Refresh interval (human-readable: "1h", "30m")
  • Custom plugin-specific fields (extensible)
• Base Debug Infrastructure: Generic BasePlugin implementation with get_custom_debug_info() override method
• Dual Rendering Support:
  • HTML plugins: Template-based overlay with CSS styling
  • Image plugins: PIL-drawn overlay with matching appearance
• Weather Plugin Custom Debug: Shows provider, API timing, location, units, and active display options (this is an example for custom fields)

Changed

• Weather Plugin: Removed custom timestamp implementation, migrated to base debug system
• All 20+ Plugins: Updated to support unified debug system
  • 9 HTML plugins: Pass device_config to render_image()
  • 11 Image plugins: Call add_debug_overlay() before returning
• Refresh System: Added generation timing and cache status tracking
• Settings UI: Added "Show Debug Info" checkbox to device settings