feat: add startup and behavior settings

[CalvinAllen]

Summary

• Add Settings tab with four configurable options:
  • Launch on startup - registers app in Windows startup registry
  • Start minimized - start hidden to system tray
  • Minimize to tray - hide to tray instead of minimizing to taskbar
  • Close to tray - hide to tray instead of closing the app
• Add SettingsService for persisting settings via JSON file in %LOCALAPPDATA%\CodingWithCalvin.VSToolbox
• Override ToggleSwitch accent color to match purple theme
• Compact settings UI layout

Closes #2

Test plan

• Toggle "Launch on startup" ON, verify registry entry created in HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
• Toggle "Launch on startup" OFF, verify registry entry removed
• Toggle "Start minimized" ON, restart app, verify it starts hidden to tray
• Toggle "Minimize to tray" ON, click minimize, verify hides to tray
• Toggle "Minimize to tray" OFF, click minimize, verify minimizes to taskbar
• Toggle "Close to tray" ON, click close, verify hides to tray
• Toggle "Close to tray" OFF, click close, verify app exits
• Verify settings persist across app restarts