feat: implement retro terminal Streak Card UI for Issue #51

[Souma-122s]

Description

I have built the frontend UI for the Streak Card

Note: The original issue description mentioned a StreakCard.jsx component, but after reviewing the codebase, I saw the project is built with HTML/CSS/JS. I adopted the solution to match the current stock perfectly.

Because the backend data for streaks is currently blocked by Issue #7, I built this UI using dummy data. It is fully styled with the requested retro-terminal theme and includes id tags on the text elements so the backend team can easily inject the live data once the API is ready.

Linked Issue

Fixes #51

Changes Made

• Added structural HTML for the streak card in frontend/leaderboard.html.
• Added retro terminal CSS matching the global theme in frontend/styles/main.css.
• Configured dynamic CSS classes (badge-completed, badge-pending) for easy JavaScript toggling later.

Type of Change

• Bug fix
• New feature
• UI/Visual update
• Documentation update
• Refactor

Testing

• Tested locally
• Tested on mobile viewport (if applicable)
• No console errors introduced

Checklist

• [ x ] My code follows the project's coding style
• [ x ] I have formatted my code locally using Prettier
• [ x ] I have performed a self-review of my code
• [ x ] My changes generate no new warnings or errors
• [ ] I have updated documentation if required
• [ x ] I have linked the relevant issue

Screenshots / Screen Recording

[github-actions]

Hello! Thank you for your first Pull Request!
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