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/app/components/JourneyForm.tsx

@@ -6,6 +6,7 @@ import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
 import { Label } from '@/components/ui/label'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
+import { DatePicker } from '@/components/ui/date-picker'
 import { Calculator, Download } from 'lucide-react'
 
 interface JourneyFormProps {
@@ -18,8 +19,8 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
   const [formData, setFormData] = useState({
     username: '',
     password: '',
-    startDate: new Date().toISOString().slice(0, 10),
-    endDate: new Date().toISOString().slice(0, 10),
+    startDate: new Date(),
+    endDate: new Date(),
     startKm: '',
     endKm: '',
     vehicleRegistration: '4SH1148',
@@ -31,9 +32,24 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault()
     setError('')
+
+    // Validate date range
+    if (formData.startDate > formData.endDate) {
+      setError('Datum od musí být před nebo stejný jako Datum do')
+      return
+    }
+
     setLoading(true)
 
     try {
+      // Format dates as YYYY-MM-DD in local timezone
+      const formatLocalDate = (date: Date) => {
+        const year = date.getFullYear()
+        const month = String(date.getMonth() + 1).padStart(2, '0')
+        const day = String(date.getDate()).padStart(2, '0')
+        return `${year}-${month}-${day}`
+      }
+
       const response = await fetch(`${API_URL}/api/calculate`, {
         method: 'POST',
         headers: {
@@ -42,8 +58,8 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
         body: JSON.stringify({
           username: formData.username,
           password: formData.password,
-          start_date: formData.startDate,
-          end_date: formData.endDate,
+          start_date: formatLocalDate(formData.startDate),
+          end_date: formatLocalDate(formData.endDate),
           start_km: parseInt(formData.startKm),
           end_km: parseInt(formData.endKm),
           vehicle_registration: formData.vehicleRegistration,
@@ -71,6 +87,14 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
   const handleExport = async () => {
     setLoading(true)
     try {
+      // Format dates as YYYY-MM-DD in local timezone
+      const formatLocalDate = (date: Date) => {
+        const year = date.getFullYear()
+        const month = String(date.getMonth() + 1).padStart(2, '0')
+        const day = String(date.getDate()).padStart(2, '0')
+        return `${year}-${month}-${day}`
+      }
+
       const response = await fetch(`${API_URL}/api/export/excel`, {
         method: 'POST',
         headers: {
@@ -79,8 +103,8 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
         body: JSON.stringify({
           username: formData.username,
           password: formData.password,
-          start_date: formData.startDate,
-          end_date: formData.endDate,
+          start_date: formatLocalDate(formData.startDate),
+          end_date: formatLocalDate(formData.endDate),
           start_km: parseInt(formData.startKm),
           end_km: parseInt(formData.endKm),
           vehicle_registration: formData.vehicleRegistration,
@@ -94,7 +118,7 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
       const url = window.URL.createObjectURL(blob)
       const a = document.createElement('a')
       a.href = url
-      a.download = `journeybook_${formData.startDate}_${formData.endDate}.xlsx`
+      a.download = `journeybook_${formData.startDate.toISOString().slice(0, 10)}_${formData.endDate.toISOString().slice(0, 10)}.xlsx`
       a.click()
     } catch (err: any) {
       setError(err.message)
@@ -138,23 +162,19 @@ export default function JourneyForm({ onDataCalculated, setLoading, onFormDataCh
         <div className="grid grid-cols-2 gap-4">
           <div className="space-y-2">
             <Label htmlFor="startDate">Datum od</Label>
-            <Input
-              id="startDate"
-              type="date"
-              required
+            <DatePicker
               value={formData.startDate}
-              onChange={(e) => setFormData({ ...formData, startDate: e.target.value })}
+              onChange={(date) => date && setFormData({ ...formData, startDate: date })}
+              placeholder="Vyberte datum od"
             />
           </div>
 
           <div className="space-y-2">
             <Label htmlFor="endDate">Datum do</Label>
-            <Input
-              id="endDate"
-              type="date"
-              required
+            <DatePicker
               value={formData.endDate}
-              onChange={(e) => setFormData({ ...formData, endDate: e.target.value })}
+              onChange={(date) => date && setFormData({ ...formData, endDate: date })}
+              placeholder="Vyberte datum do"
             />
           </div>
         </div>