feat: add dark mode toggle with system preference detection

[Nareshkumawat-star]

Feature Request

DevCard currently has no dark mode support. Most users expect apps to respect their OS-level dark/light preference and provide a manual override.

Proposed Solution

• Detect prefers-color-scheme on load and apply appropriate theme
• Add a sun/moon toggle button in the header/settings screen
• Persist user preference in AsyncStorage (mobile) and localStorage (web)
• Apply dark theme CSS variables / React Native StyleSheet overrides

Affected Platforms

• Web
• Mobile (React Native)

Acceptance Criteria

• Dark mode activates automatically based on OS preference
• Toggle persists across sessions
• All screens render correctly in dark mode

[github-actions]

Hi @Nareshkumawat-star,

Thanks for opening this issue.

Please reply with one of the following areas so the issue can be routed to the appropriate team member:

- /backend
- /web
- /mobile
- /devops

Once an area is selected, the corresponding label will be added automatically.

[Nareshkumawat-star]

/web
/mobile

[Nareshkumawat-star]

/web

[github-actions]

The issue has been classified as web.

@ShantKhatri, please review and triage this issue when available.

The web label has been applied and the issue has been routed accordingly.

[Nareshkumawat-star]

/mobile