Add CSV, JSON, and Markdown submission downloads

[cuevaio]

Summary

• Move the existing CSV export into the View submissions toolbar.
• Add JSON downloads using the existing export dataset.
• Add escaped Markdown files packaged as a ZIP with numbered, sanitized filenames.
• Harden CSV output against spreadsheet formulas and multiline values.
• Load JSZip only when a Markdown archive is requested.

Implementation

All formats use the existing judging.results-protected export query. This PR does not change the Convex API, database schema, authorization, or exported dataset.

Verification

• npm run build
• Focused ESLint on all changed source files
• CSV escaping checked for empty, whitespace, formula, and multiline cells
• JSZip confirmed as a separate lazy-loaded chunk
• Diff checked for local paths, deployment identifiers, test scaffolding, and unrelated skill changes