Final working solution: shadcn date picker with timezone fix

- Implemented shadcn/ui date picker with Czech localization
- Added month/year dropdown navigation for easy date selection
- Fixed critical timezone bug causing "No valid days found" error
  - Changed from toISOString() to local date formatting
  - Dates now correctly sent as 2025-01-01 instead of 2024-12-31
- Calendar auto-closes after date selection
- All features tested and working:
  - Journey calculation with correct date ranges
  - "Vyplnit na web" button visible and functional
  - Excel export working
  - Backend successfully processes January 2025 data

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/package.json

@@ -14,20 +14,26 @@
   "description": "",
   "dependencies": {
     "@radix-ui/react-label": "^2.1.7",
+    "@radix-ui/react-popover": "^1.1.15",
+    "@radix-ui/react-select": "^2.2.6",
     "@radix-ui/react-slot": "^1.2.3",
-    "@tailwindcss/postcss": "^4.1.14",
     "@types/node": "^24.7.1",
     "@types/react": "^19.2.2",
-    "autoprefixer": "^10.4.21",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "date-fns": "^4.1.0",
     "lucide-react": "^0.545.0",
     "next": "^15.5.4",
-    "postcss": "^8.5.6",
     "react": "^19.2.0",
+    "react-day-picker": "^9.11.1",
     "react-dom": "^19.2.0",
     "tailwind-merge": "^3.3.1",
-    "tailwindcss": "^4.1.14",
     "typescript": "^5.9.3"
+  },
+  "devDependencies": {
+    "autoprefixer": "^10.4.21",
+    "postcss": "^8.5.6",
+    "tailwindcss": "^3.4.18",
+    "tailwindcss-animate": "^1.0.7"
   }
 }