feat: implement dark/light mode with system detection

- Install and configure next-themes for theme management
- Add ThemeProvider wrapper component with suppressHydrationWarning
- Define light theme CSS variables (off-white backgrounds, green accents)
- Define dark theme CSS variables (hacker theme: terminal blacks, matrix green)
- Update scrollbar styles for both themes
- Add CRT scanline/glow effects (dark mode only)
- Create ThemeToggle component with system/light/dark cycling
- Add theme toggle to Toolbox sidebar
- Update CandleChart to apply theme-specific colors
- Chart colors update dynamically on theme change
- All components use CSS variables for theme compatibility
- FOUC prevention via next-themes inline script

package.json

@@ -28,6 +28,7 @@
     "lightweight-charts": "^4.2.3",
     "lucide-react": "^0.563.0",
     "next": "^16.1.6",
+    "next-themes": "^0.4.6",
     "papaparse": "^5.5.3",
     "postcss": "^8.5.6",
     "react": "^19.2.4",