fix(frontend): upgrade react-day-picker to v9 for React 19 support

[xinkaichen97]

Summary

• npm install fails on a fresh clone with ERESOLVE: react-day-picker@^8.10.1's peerDependencies cap at React 18, while this project requires react@^19.2.0.
• Bump react-day-picker to ^9.14.0, whose peer range is >=16.8.0 (no ceiling), so it actually supports React 19.
• Update calendar.tsx's classNames/components props to v9's renamed API (caption→month_caption, head_row→weekdays, day→day_button, day_selected→selected, single Chevron component replacing IconLeft/IconRight, etc.) — v9 doesn't recognize the old key names, so leaving them would silently drop calendar styling.

Test plan

• npm install succeeds with no --legacy-peer-deps/--force
• tsc -b — no type errors
• vite build — succeeds
• Manually exercised the date-range calendar widget in the browser — range selection, styling, and nav chevrons render correctly