Add comprehensive dark mode instructions to built-in help documentation

[Copilot]

This PR addresses the long-standing request to document dark mode enablement by moving the latest instructions from GitHub issue #677 into the built-in help documentation.

What's Changed

Added a new "Dark Mode" section to rednotebook/help.py that provides comprehensive, platform-specific instructions for enabling dark themes in RedNotebook:

Linux (GNOME/GTK environments)

• GNOME Tweaks method: Step-by-step instructions for using the graphical interface
• Environment variables: Command-line approach using GTK_THEME
• Support for both older systems (Adwaita-dark) and newer Debian 13/Trixie (HighContrastInverse)

Flatpak installations

• Flatseal GUI method: Using the permissions manager for easy configuration
• Command-line method: Using flatpak override for users who prefer terminal commands

General cross-platform method

• Environment variable approach that works across Linux, macOS, and Windows
• Documentation of common dark theme names and system variations

Background

The instructions are based on community contributions in GitHub issue #677, particularly from contributors who identified working solutions for different platforms and installation methods. This consolidates scattered knowledge into the official documentation where users can easily find it.

Technical Details

• The new section is positioned before the existing "Appearance on Windows" section for logical flow
• All existing help documentation structure is preserved
• HTML generation continues to work correctly with proper table of contents integration
• Updated TODO.md to mark this documentation task as completed

Users can now access these instructions directly through RedNotebook's built-in help system instead of searching through GitHub issues.

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