fix(dates): use AdminDatePicker everywhere + mode-aware default placeholder

- The project create-modal start/end dates used raw <input type="date">;
  converted to <AdminDatePicker> so every date field in the app now goes
  through the shared component (react-datepicker on desktop, native on touch).
- AdminDatePicker now defaults its placeholder by mode (date 'dd.mm.rrrr',
  month 'mm.rrrr', time 'hh:mm') so empty date fields always show a format
  hint; explicit placeholders (e.g. 'Od'/'Do' range filters) still override.
- Dropped WarehouseReceiptForm's ad-hoc 'dd.mm.yyyy' so it inherits the
  unified Czech default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components/AdminDatePicker.tsx

@@ -58,6 +58,7 @@ interface NativeInputProps {
   minDate?: string;
   maxDate?: string;
   disabled?: boolean;
+  placeholder?: string;
 }
 
 const modeToInputType: Record<string, string> = {
@@ -73,6 +74,7 @@ function NativeInput({
   minDate,
   maxDate,
   disabled,
+  placeholder,
 }: NativeInputProps) {
   const type = modeToInputType[mode] || "date";
   // For time inputs, min/max must be in HH:mm format, not date format
@@ -95,6 +97,7 @@ function NativeInput({
       value={value || ""}
       onChange={(e) => onChange(e.target.value)}
       className="admin-form-input"
+      placeholder={placeholder}
       required={required}
       disabled={disabled}
       min={minProp}
@@ -126,6 +129,12 @@ export default function AdminDatePicker({
 }: AdminDatePickerProps) {
   const useNative = isTouchDevice();
 
+  // Mode-aware default placeholder so empty date fields always show a format
+  // hint (callers can still override, e.g. "Od"/"Do" on range filters).
+  const effectivePlaceholder =
+    placeholder ??
+    (mode === "time" ? "hh:mm" : mode === "month" ? "mm.rrrr" : "dd.mm.rrrr");
+
   if (useNative) {
     return (
       <NativeInput
@@ -136,6 +145,7 @@ export default function AdminDatePicker({
         minDate={minDate}
         maxDate={maxDate}
         disabled={disabled}
+        placeholder={effectivePlaceholder}
       />
     );
   }
@@ -181,7 +191,7 @@ export default function AdminDatePicker({
   const customInput = (
     <CustomInput
       required={required}
-      placeholder={placeholder}
+      placeholder={effectivePlaceholder}
       disabled={disabled}
     />
   );
@@ -191,7 +201,7 @@ export default function AdminDatePicker({
     onChange: handleChange,
     locale: "cs",
     customInput,
-    placeholderText: placeholder,
+    placeholderText: effectivePlaceholder,
     minDate: parseMinMax(minDate),
     maxDate: parseMinMax(maxDate),
     popperPlacement: "bottom-start" as const,