feat: add dark mode toggle

[idiottrader]

Summary

This PR adds dark mode support to SoroSave as requested in #31.

Changes

• ThemeProvider.tsx: React Context for theme management with localStorage persistence
• ThemeToggle.tsx: Toggle button with sun/moon icons
• Navbar.tsx: Updated with ThemeToggle and dark mode styles
• providers.tsx: Integrated ThemeProvider
• layout.tsx: Added suppressHydrationWarning for Next.js
• globals.css: Added dark mode CSS variables
• tailwind.config.js: Enabled darkMode: 'class'

Features

• ✅ Toggle between light/dark modes
• ✅ Persist user preference in localStorage
• ✅ Respect system preference on first load
• ✅ Smooth transition animations
• ✅ All existing components styled for both modes

Testing

• Theme toggle button visible in navbar
• Clicking toggle switches between light/dark modes
• Theme preference persists after page reload
• All components render correctly in both modes
• No hydration errors in console

Closes #31