feat: implement full dark mode across mobile app (#226)

[Ummi-001]

Closes #226

What I did

• Updated mobile/app/_layout.tsx overlay to use theme colors
• Updated mobile/app/(tabs)/index.tsx to use theme colors instead of hardcoded values
• Created mobile/components/ThemeToggle.tsx for manual dark/light mode toggle
• Dark mode uses existing ThemeContext with DARK_COLORS and LIGHT_COLORS
• Toggle persists preference using useAuthStore
• Supports system preference and manual override
• No contrast issues between light and dark modes

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