Dark Mode & UI Improvements

This PR adds dark mode support to Devise views and improves overall UI organization.

Dark Mode Support

• Added dark mode styling across all Devise views
• Enhanced error_messages partial with proper dark mode colors
• Updated password reset views with dark mode compatibility
• Fixed styling in all Devise templates to support dark mode
• Add dark mode preference detection on top of app preference

Improved Table Navigation

• Converted "My Assignments" List/Table views to a tabbed interface
• Created scrollable containers that utilize available viewport height
• Applied sticky positioning to table headers in Table View
• Made date headers in List View sticky as well
• Added appropriate background colors to ensure sticky headers remain visible

User Profile Enhancements

• Grouped appearance settings (Theme Preference and Profile Color)
• Added better labels and helpful descriptions for settings
• Created a collapsible "Change Password" section (hidden by default)
• Made "More Languages Team only" section collapsible and collapsed by default
• Implemented measures to prevent browser autocomplete on password fields