feat: Replace default browser scrollbar with custom themed scrollbar #151

Summary

The website uses the default browser scrollbar which visually clashes with DevCard's
theme. A custom CSS scrollbar matching the site's color palette would improve
UI consistency and overall polish.

Context

On Chromium-based browsers, the default scrollbar is visually heavy and does not
blend with the dark/themed UI of DevCard. This is especially noticeable on the
DevCard profile view screen. A lightweight CSS-only fix using ::-webkit-scrollbar
pseudo-elements (Chromium/Safari) and scrollbar-color (Firefox) can resolve this
without any JavaScript.

Tasks

• Identify the theme accent colors / CSS variables from the existing Tailwind config or app.css
• Implement custom scrollbar styles in the global stylesheet
• Test appearance across different browsers