feat(dashboard): add json export functionality

[Aspirant200715]

Summary

Added a new "Export JSON" feature to the dashboard, allowing users to download their coding activity, goals, and metric snapshots in a highly structured programmatic format, while safely falling back if the database is disconnected.

Closes #1030

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Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

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Changes Made

• Created a new exportJSON function inside the ExportButton component.
• Implemented a unified fetch that combines GitHub metrics (commits, PRs) with full database extracts (/api/user/data-export).
• Added graceful fallback logic so the application will not crash if Supabase credentials are missing locally.
• Added a styled "Export JSON" button to the dashboard UI matching the existing CSV/PDF buttons.
• Formatted the resulting output into a clean JSON structure and handled client-side Blob downloading.

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How to Test

Steps for the reviewer to verify this works:

1. Run the local development server (pnpm dev)
2. Log in using GitHub OAuth and navigate to the Dashboard.
3. Click the new "Export JSON" button located at the top right of the dashboard.
4. Open the downloaded .json file and verify that the data correctly maps your GitHub commits, PRs, and (if connected) your Supabase goals and streak history.

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Screenshots (if UI change)

The UI introduces a third "Export JSON" button perfectly matching the flex layout of the existing export buttons.

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Checklist

• Linked issue in summary
• pnpm run lint passes locally
• No TypeScript errors (pnpm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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Accessibility Checklist

• Proper keyboard navigation tested
• Responsive UI verified
• Accessibility labels added where needed

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Additional Notes

The JSON export route was heavily tested for edge cases. It is intentionally designed to bypass strict errors when /api/goals fails, allowing users to export their GitHub contributions still even if they are in an incomplete local dev environment.

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[github-actions]

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[Priyanshu-byte-coder]

Hi! Your PR has merge conflicts with the current main branch. Please rebase your branch on latest main and resolve the conflicts:

git fetch upstream
git rebase upstream/main
# resolve any conflicts
git push --force-with-lease origin your-branch

Once resolved, we'll merge it in. Thanks!

[Aspirant200715]

Hi @Priyanshu-byte-coder, I have resolved the conflicts as asked pls check once