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/components/ui/date-picker.tsx

@@ -0,0 +1,64 @@
+"use client"
+
+import * as React from "react"
+import { format } from "date-fns"
+import { cs } from "date-fns/locale"
+import { CalendarIcon } from "lucide-react"
+
+import { cn } from "@/lib/utils"
+import { Button } from "@/components/ui/button"
+import { Calendar } from "@/components/ui/calendar"
+import {
+  Popover,
+  PopoverContent,
+  PopoverTrigger,
+} from "@/components/ui/popover"
+
+interface DatePickerProps {
+  value?: Date
+  onChange?: (date: Date | undefined) => void
+  placeholder?: string
+}
+
+export function DatePicker({ value, onChange, placeholder = "Pick a date" }: DatePickerProps) {
+  const [date, setDate] = React.useState<Date | undefined>(value)
+  const [open, setOpen] = React.useState(false)
+
+  React.useEffect(() => {
+    setDate(value)
+  }, [value])
+
+  const handleSelect = (selectedDate: Date | undefined) => {
+    setDate(selectedDate)
+    onChange?.(selectedDate)
+    setOpen(false)
+  }
+
+  return (
+    <Popover open={open} onOpenChange={setOpen}>
+      <PopoverTrigger asChild>
+        <Button
+          variant="outline"
+          className={cn(
+            "w-full justify-start text-left font-normal",
+            !date && "text-muted-foreground"
+          )}
+        >
+          <CalendarIcon className="mr-2 h-4 w-4" />
+          {date ? format(date, "PPP", { locale: cs }) : <span>{placeholder}</span>}
+        </Button>
+      </PopoverTrigger>
+      <PopoverContent className="w-auto p-0" align="start">
+        <Calendar
+          mode="single"
+          selected={date}
+          onSelect={handleSelect}
+          captionLayout="dropdown"
+          startMonth={new Date(1900, 0)}
+          endMonth={new Date()}
+          locale={cs}
+        />
+      </PopoverContent>
+    </Popover>
+  )
+}