feat: Add dark mode toggle with system preference detection

[derekliutao-cmd]

Summary

Closes #10

Implements dark mode toggle with system preference detection using next-themes.

Changes

src/app/globals.css — Added .dark class with dark mode CSS variable overrides (dark backgrounds, light foregrounds, adjusted primary/muted/border colors)

src/components/ui/theme-provider.tsx (new) — ThemeProvider wrapper around next-themes, configured with attribute="class", defaultTheme="system", enableSystem, disableTransitionOnChange

src/components/ui/theme-toggle.tsx (new) — Sun/Moon toggle button with:

• Hydration-safe rendering (mounted check)
• Smooth rotation/scale animation between icons
• Accessible aria-label

src/components/providers.tsx — Wrapped QueryClientProvider with ThemeProvider

src/components/ui/sidebar.tsx — Added ThemeToggle next to Stellar Testnet indicator in sidebar footer

Acceptance Criteria Met

• Dark mode CSS variable overrides in globals.css
• ThemeProvider using next-themes (attribute="class", system default)
• Toggle button (sun/moon) in sidebar footer, accessible
• System preference detected on first load (enableSystem)
• suppressHydrationWarning on html element (already in layout.tsx)
• No flash of wrong theme (disableTransitionOnChange + suppressHydrationWarning)

Note

Requires adding next-themes to package.json dependencies (^0.3.0)