Implement dark/light theme switcher

[Konstantysz]

Problem

Fixed light theme may strain eyes in low-light environments. Many users prefer dark mode.

Proposal

Add theme switcher in menu bar (View → Theme → Dark/Light).

Implementation

• Define color palettes in Theme.h (struct with colors for background, text, borders, etc.)
• Create ThemeDark and ThemeLight presets
• Apply via ImGui::StyleColorsDark() / ImGui::StyleColorsLight() + custom tweaks
• Save preference to settings.json (add Settings persistence class)

Design Notes

• Dark theme: Dark gray background, light text, blue accents
• Light theme: White background, dark text, blue accents

Acceptance Criteria

• Theme persists across restarts
• All UI elements are readable in both themes