Support OS theme auto-sync

[deedee-code]

Description

This PR implements system-level theme detection and prevents the flash of unstyled content (FOUC) on load:

• Updated ThemeProvider: Enabled enableSystem and set defaultTheme="system" to respect OS preferences on first load.
• Injected Theme Script: Added a blocking inline script in the root HTML to apply the correct theme class before the page renders, eliminating the "theme flash."
• Improved Theme Sync: Refactored the theme detection logic to correctly handle the 'system' value from localStorage in sync with the user's OS settings.

Closes: #425

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