fix: improve dashboard responsiveness for mobile and tablet screens

[Kokila-chandrakar]

Closes #1404

Problem

Dashboard components, cards, and layout sections were not fully optimized for smaller screen sizes. Issues included cramped spacing, overflow problems, and fixed layouts that didn't adapt to mobile viewports.

Changes

src/app/dashboard/page.tsx

• Outer container: Reduced padding on mobile (px-3 py-5) with progressive scaling (sm:px-6 sm:py-8 lg:px-8)
• Quick actions: Buttons stack vertically on mobile, row layout on sm+
• Section headings: Scaled from text-xl (mobile) to text-2xl (desktop)
• Section accent bars: Scaled from h-7 (mobile) to h-8 (desktop)
• Section margins: Responsive spacing (mt-10 sm:mt-14)
• Scroll margins: Adjusted for mobile (scroll-mt-20 sm:scroll-mt-28)
• Contribution graphs: Added horizontal scroll on small screens (overflow-x-auto -mx-3 px-3 sm:mx-0 sm:px-0)

src/components/DashboardHeader.tsx

• Header padding: Progressive scaling (p-4 sm:p-5 md:p-6) for better mobile fit

Notes

• No features were removed or modified — all changes are CSS/layout only
• CustomizableDashboard, lastSynced localStorage persistence, and night-owl/early-bird badge detection are fully intact
• All existing functionality remains unchanged

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[github-actions]

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