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/pages/Projects.tsx

@@ -8,6 +8,7 @@ import { motion } from "framer-motion";
 import ConfirmModal from "../components/ConfirmModal";
 import FormModal from "../components/FormModal";
 import FormField from "../components/FormField";
+import AdminDatePicker from "../components/AdminDatePicker";
 
 import { formatDate, czechPlural } from "../utils/formatters";
 import SortIcon from "../components/SortIcon";
@@ -530,23 +531,21 @@ export default function Projects() {
 
           <div className="admin-form-row">
             <FormField label="Začátek">
-              <input
-                type="date"
+              <AdminDatePicker
+                mode="date"
                 value={createForm.start_date}
-                onChange={(e) =>
-                  setCreateForm({ ...createForm, start_date: e.target.value })
+                onChange={(val) =>
+                  setCreateForm({ ...createForm, start_date: val })
                 }
-                className="admin-form-input"
               />
             </FormField>
             <FormField label="Konec">
-              <input
-                type="date"
+              <AdminDatePicker
+                mode="date"
                 value={createForm.end_date}
-                onChange={(e) =>
-                  setCreateForm({ ...createForm, end_date: e.target.value })
+                onChange={(val) =>
+                  setCreateForm({ ...createForm, end_date: val })
                 }
-                className="admin-form-input"
               />
             </FormField>
           </div>

src/admin/pages/WarehouseReceiptForm.tsx

@@ -378,7 +378,6 @@ export default function WarehouseReceiptForm() {
                       delivery_note_date: val,
                     }))
                   }
-                  placeholder="dd.mm.yyyy"
                 />
               </FormField>
             </div>