feat: add Export to PDF action on public profile page (#2604)

[aaniya22]

closes #2604

Summary

Adds a "Print Profile / Export to PDF" action on the public profile page. Clicking the button triggers the browser's native window.print(), with @media print CSS overrides that hide navigation, share, and footer elements so the printed output shows a clean, card-based metrics layout.

Closes #2604

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Type of Change

• ✨ New feature (non-breaking change that adds functionality)

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What Changed

• Added src/components/ExportPdfButton.tsx — client button that triggers window.print()
• Wired ExportPdfButton into src/app/u/[username]/page.tsx, placed alongside the compare buttons
• Added print:hidden to the Share section, Powered-by-DevTrack footer, and other non-essential UI so they're excluded from the printed/PDF output
• Added @media print rules in src/app/globals.css to strip shadows, flatten borders on card elements (.surface-card, [class*="rounded-2xl"]), and prevent cards from breaking across pages

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How to Test

1. Visit any public profile page, e.g. /u/<username>
2. Click the new "Export to PDF" button near the top of the profile
3. In the print preview dialog, confirm the layout shows only the profile cards — no nav, share buttons, or footer
4. Save as PDF and confirm cards render cleanly without being cut across pages

Expected result: A clean, print-ready view of the profile stats with no interactive UI elements, matching the layout described in the issue.

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed
• Tested on mobile / responsive layout

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Additional Context

No external dependencies added — uses native window.print() and CSS @media print per the issue's acceptance criteria.

[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

[aaniya22]

The 2 failing tests (in ProfileQrModal.test.tsx and ShortcutsModal.test.tsx) are pre-existing failures on main, unrelated to this PR. I verified by running the same test files directly against main before my changes — they fail there too with identical errors. Nothing in this PR touches those files. Flagging here in case it's worth tracking as a separate issue.