Add downloadable table feature with CSV, JSON, and Excel export

[rohanbalkondekar]

Summary

• Adds a Download button next to "Clear Filters" in the leaderboard controls
• Two-column panel with radio buttons for scope (Current View / Full Data) and checkboxes for format (CSV, JSON, Excel) — users can select multiple formats at once
• Export values are numeric (not formatted strings) for proper use in spreadsheets and programmatic consumption
• Full Data export includes all models regardless of metadata availability
• Added xlsx dependency (SheetJS) for Excel export

Files changed

• package.json / package-lock.json — added xlsx dependency
• src/Table/downloadTable.js — new utility: data builders + download triggers
• src/Table/CSVTable.jsx — download panel UI with state management
• src/App.css — styles for action buttons row and download panel

Test plan

• Click Download button — panel opens with radio + checkbox options
• Select "Current View" + CSV — verify exported rows/columns match visible table
• Select "Full Data" + JSON — verify all models included with numeric values
• Select multiple formats — verify all selected formats download
• Apply filters, then download Current View — verify filters are respected
• Click outside panel — verify it closes
• Verify buttons are centered below the filter checkboxes