Implement Theme System with Dark/Light Mode Support

[StephenTheodore]

Goal

Implement comprehensive theme system with dark/light mode switching as specified in UI requirements.

Issue Background

README.md specifies "Dark mode preferred or selectable option" but current implementation only has basic dark theme hardcoded in AnalyzeView.

Requirements

• Create theme resource dictionaries (Dark.axaml, Light.axaml)
• Implement theme switching service
• Add theme toggle in SettingsView
• Update all views to use theme resources instead of hardcoded colors
• Persist theme preference in settings
• Support system theme detection

Theme Resources Needed

• Colors: Background, foreground, accent, border colors
• Brushes: Gradients, patterns for UI elements
• Styles: Button, TextBlock, Border styles for consistency
• Icons: Theme-appropriate icons and symbols

Implementation Components

• IThemeService interface for theme management
• ThemeService implementation with persistence
• Theme resource dictionaries
• Settings integration for user preference
• System theme detection (optional)

Acceptance Criteria

• Theme resource dictionaries created
• Theme switching service implemented
• Settings UI for theme selection working
• All views use theme resources
• Theme preference persistence working
• Smooth theme switching transitions
• System theme detection (if possible)

Priority

Medium - UI enhancement requirement

Labels

enhancement, ui, theming, dark-mode, light-mode, settings