[Feat]: Add persistent dark theme toggle with localStorage state caching

[Siddh2024]

Description

This Pull Request introduces a native, state-persistent Dark Theme option to the Devpath platform. This enhancement improves visual accessibility and comfort during low-light operation while strictly preserving the repository's original global typography and structural core components.

Proposed Changes

• Structural Theme Switcher: Integrated an intuitive, accessible theme toggle element featuring sharp inline SVG icons into the main navigation layout.
• Semantic Styling Variables: Refactored hardcoded interface colors into dynamic CSS variables (--bg-primary, --text-main, --card-bg) supporting smooth runtime transitions.
• State Persistence Caching: Implemented browser client-side localStorage hook mechanisms to ensure the user's theme selection remains intact across page refreshes and subsequent sessions.

Related Issue

Closes #318

Verification Checklist

• Verified seamless runtime theme swapping without visual layout shifts.
• Confirmed selected theme state persists correctly using browser inspector caching tools.
• Audited typography contrast levels across all layout blocks to ensure optimal accessibility.

[vercel]

@Siddh2024 is attempting to deploy a commit to the komalsony234-1530's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Siddh2024]

@komalharshita , please approve this pr ma'am.