Add dark mode to document editor and simplify editor layout

- Add dark mode support to documents/edit.html.erb (navbar, buttons, dropdowns)
- Simplify editor.html.erb layout by removing duplicate sidebar
- Update content linking partials with dark mode styles
- Fix dark mode colors for writing tips card on prompts page

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: drusepth