feat(dashboard): add dark/light mode toggle

[Uday9909]

Drafted in GOOD_FIRST_ISSUES.md; opening as a tracked issue.

Add a toggle switching the dashboard between light and dark theme, persisted in localStorage.

Files: dashboard/src/App.jsx, dashboard/src/index.css

AC:

• Toggle button in header/navbar
• Clicking switches themes
• Preference saved to localStorage, restored on reload
• All components render correctly in both modes

[sharancreates]

I want to go for it. Please assign it to me @Uday9909

[Uday9909]

@sharancreates go for it!