feat(theme): add customizable theme system with live switching

[echobt]

Summary

Add comprehensive theme support for the TUI with real-time theme switching capability.

Changes

Core Theme System

• Add ThemeConfig struct with serde support for persistence
• Define 4 built-in themes: dark, light, ocean_dark, monokai
• Add ThemeColors with from_name() lookup and available_themes() list
• Add MarkdownTheme variants for each theme with full color customization

TUI Integration

• Add ThemeSelectorModal for interactive theme selection
• Add 'Change Theme' command to command palette (Settings category)
• Extend AdaptiveColors with theme support methods
• Add theme state to AppState with set_theme() method

User Experience

• Update rendering pipeline to use theme from state
• Add themed markdown rendering for conversation display
• Live switching: Theme changes apply instantly to the entire UI
• Persistence: Last selected theme is saved and restored on startup

Files Changed

• src/cortex-core/src/markdown/theme.rs - MarkdownTheme variants
• src/cortex-core/src/style.rs - ThemeColors definitions
• src/cortex-tui/src/modal/theme.rs - ThemeSelectorModal
• src/cortex-tui/src/ui/colors.rs - AdaptiveColors theme support
• Various rendering and state files

Testing

• Code compiles with cargo check
• Theme switching works in real-time
• Theme persists across sessions