Fix build errors in Calendar and Database Seed script

[google-labs-jules]

Fixed type errors preventing the project from building:

• Updated src/components/ui/calendar.tsx to use Chevron component for navigation icons, compatible with react-day-picker v9.
• Updated src/lib/seed-db.ts to use correct Firebase Firestore Modular SDK syntax for creating document references.
• Verified fixes with npm run typecheck.

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PR created automatically by Jules for task 2273634534669475093 started by @ViVaLaDaniel

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