feat: add copy profile link button

[amritbej]

Summary

Added a Copy Link button beside the Get DevCard CTA on the profile page. The button copies the current profile URL to the clipboard using the Clipboard API and shows transient success/failure notifications. Also added a fallback message for browsers where the Clipboard API is unavailable.

Closes #15

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Type of Change

• Bug fix
• New feature
• Refactor (no functional change)
• UI / Design change
• Tests only
• Documentation
• Infrastructure / DevOps
• Security

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What Changed

• Added a Copy Link button near the Get DevCard CTA in apps/web/src/routes/u/[username]/+page.svelte
• Implemented clipboard copy functionality using navigator.clipboard.writeText(window.location.href)
• Added transient success/failure notifications and unsupported Clipboard API fallback handling

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How to Test

1. Run the frontend locally using pnpm dev:web

2. Open a profile page such as http://localhost:5174/u/devcard-demo

3. Click the Copy Link button and verify:

   • The URL is copied successfully
   • A success message appears
   • A fallback/error message appears if Clipboard API is unavailable

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Checklist

• My code follows the project's coding style (pnpm -r run lint passes).
• TypeScript compiles without errors (pnpm -r run typecheck).
• I have added or updated tests for the changes I made.
• All tests pass locally (pnpm -r run test).
• I have updated documentation where necessary.
• No new console.log or debug statements left in the code.
• Breaking changes are documented in this PR description.

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UI change showing the new Copy Link button and notification behavior.

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Additional Context

Production build completed successfully using pnpm build:web.