feat: add dark mode theme toggle with next-themes (#118)

[Awointa]

Summary

Implements full dark/light/system theme support across the app using next-themes, replacing the previous Zustand themeStore. Preference persists across reloads with no flash of unstyled content.

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What changed

Dependencies

• Installed next-themes
• Configured Tailwind darkMode: 'class'

frontend/src/app/layout.tsx

• ThemeProvider wired in with system as the default
• Prevents FOUC on reload via next-themes built-in attribute injection

ThemeToggle (new component)

• 3-step cycle: Light → Dark → System
• Sun / Moon / Monitor icons per mode
• Exposed in the Header action area

Removed

• Zustand themeStore — replaced entirely by next-themes useTheme

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Acceptance criteria

Criteria	Status
Light, dark, and system modes supported	✅
Preference persists in local storage	✅
No FOUC on reload	✅
Zustand theme store removed	✅

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Checklist

• ThemeProvider configured with system default
• ThemeToggle cycles through all 3 modes
• FOUC prevented on reload
• Preference persists across sessions
• Zustand themeStore fully removed
• Ready for review

Closes #118

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