feat(ui): add multiple dashboard themes

[KrutagyaKaneria]

Summary

Implemented a complete multi-theme customization system for the DevTrack dashboard.

Added Themes

• Classic Dark
• Modern Light Blue
• Nordic Frost
• Cyberpunk / Matrix

Features

• CSS variable based theming
• Persistent theme selection using localStorage
• Theme dropdown selector
• Tailwind-compatible design tokens
• Smooth global theme switching
• Backward compatibility with existing dark mode classes

Technical Improvements

• Centralized theme registry
• Reusable semantic color tokens
• Hydration-safe theme initialization
• Minimal architectural changes

close #1675

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Please rebase on main to resolve conflicts. Run: git fetch origin && git rebase origin/main

Also note: this repo uses pnpm (not npm). Please remove package-lock.json and use pnpm-lock.yaml instead.

[Priyanshu-byte-coder]

This PR has merge conflicts in layout.tsx, globals.css, tailwind.config.ts, and package-lock.json. These files were modified on main after this PR was opened. Please rebase on the latest main to resolve the conflicts.