Unify date display to DD.MM.YYYY across the UI

Previously two formats showed up next to each other: raw ISO
(YYYY-MM-DD) where DB values were interpolated directly, and
toLocaleDateString('de-DE') output (D.M.YYYY without leading
zeros) where formatDate was used.

- formatDate / formatDateTime now build the string explicitly with
  zero-padded day and month, so dates render as DD.MM.YYYY (and
  DD.MM.YYYY HH:MM for timestamps) regardless of locale.
- Replaced the remaining raw uses (Anbieter-Admin createdAt,
  Mitarbeiter-Prozentsätze gueltig_ab, the new Benutzer tab's
  createdAt, PDF export date filters and footer).
- Native <input type=\"date\"> fields are unchanged — they require
  the ISO value internally and render localized in the picker.

public/script.js

@@ -601,9 +601,16 @@ async function loadSchaltungen() {
     }
 }
 
+// Format any ISO-like date string (YYYY-MM-DD or full timestamp) as DD.MM.YYYY
+// with leading zeros. Returns the original string unchanged if it cannot be parsed.
 function formatDate(dateString) {
+    if (!dateString) return '';
     const date = new Date(dateString);
-    return date.toLocaleDateString('de-DE');
+    if (isNaN(date.getTime())) return String(dateString);
+    const dd = String(date.getDate()).padStart(2, '0');
+    const mm = String(date.getMonth() + 1).padStart(2, '0');
+    const yyyy = date.getFullYear();
+    return `${dd}.${mm}.${yyyy}`;
 }
 
 async function showDocuments(schaltungId) {
@@ -848,7 +855,7 @@ async function loadAnbieterAdmin() {
         }
         
         anbieter.forEach(a => {
-            const createdAt = new Date(a.created_at).toLocaleDateString('de-DE');
+            const createdAt = formatDate(a.created_at);
             const tr = document.createElement('tr');
             tr.innerHTML = `
                 <td>${a.id}</td>
@@ -1844,8 +1851,15 @@ async function loadGeloeschteSchaltungen() {
 }
 
 function formatDateTime(dateTimeString) {
-    const date = new Date(dateTimeString);
-    return date.toLocaleDateString('de-DE') + ' ' + date.toLocaleTimeString('de-DE', {hour: '2-digit', minute: '2-digit'});
+    if (!dateTimeString) return '';
+    const d = new Date(dateTimeString);
+    if (isNaN(d.getTime())) return String(dateTimeString);
+    const dd = String(d.getDate()).padStart(2, '0');
+    const mm = String(d.getMonth() + 1).padStart(2, '0');
+    const yyyy = d.getFullYear();
+    const HH = String(d.getHours()).padStart(2, '0');
+    const MM = String(d.getMinutes()).padStart(2, '0');
+    return `${dd}.${mm}.${yyyy} ${HH}:${MM}`;
 }
 
 // Filter functionality
@@ -3096,7 +3110,7 @@ async function loadMitarbeiterProzentsaetzeAdmin() {
                         <div>
                             <strong>${escapeHtml(String(p.prozentsatz))}%</strong>
                             ${p.beschreibung ? `- ${escapeHtml(p.beschreibung)}` : ''}
-                            <br><small class="text-muted">Gültig ab: ${escapeHtml(p.gueltig_ab)}</small>
+                            <br><small class="text-muted">Gültig ab: ${escapeHtml(formatDate(p.gueltig_ab))}</small>
                         </div>
                         <div>
                             <button data-mp-action="delete" data-id="${p.id}" class="btn-danger">
@@ -3577,18 +3591,18 @@ function exportProvisionenToPdf() {
                 yPos += 5;
             }
             if (datumVon) {
-                doc.text(`• Von: ${new Date(datumVon).toLocaleDateString('de-DE')}`, 25, yPos);
+                doc.text(`• Von: ${formatDate(datumVon)}`, 25, yPos);
                 yPos += 5;
             }
             if (datumBis) {
-                doc.text(`• Bis: ${new Date(datumBis).toLocaleDateString('de-DE')}`, 25, yPos);
+                doc.text(`• Bis: ${formatDate(datumBis)}`, 25, yPos);
                 yPos += 5;
             }
             yPos += 5;
         }
 
         // Add generation date
-        doc.text(`Erstellt am: ${new Date().toLocaleDateString('de-DE')} um ${new Date().toLocaleTimeString('de-DE')}`, 20, yPos);
+        doc.text(`Erstellt am: ${formatDateTime(new Date())}`, 20, yPos);
         yPos += 15;
 
         // Table headers
@@ -3678,8 +3692,8 @@ function exportProvisionenToPdf() {
             // Prepare row data
             const rawData = [
                 schaltung.schaltungs_id || '',
-                schaltung.eingabe_datum ? new Date(schaltung.eingabe_datum).toLocaleDateString('de-DE') : '',
-                schaltung.schaltungs_datum ? new Date(schaltung.schaltungs_datum).toLocaleDateString('de-DE') : '',
+                schaltung.eingabe_datum ? formatDate(schaltung.eingabe_datum) : '',
+                schaltung.schaltungs_datum ? formatDate(schaltung.schaltungs_datum) : '',
                 `${schaltung.vorname || ''} ${schaltung.nachname || ''}`.trim() + (schaltung.firma ? ` (${schaltung.firma})` : ''),
                 schaltung.anbieter_name || '',
                 schaltung.tarif_name || '',
@@ -3780,7 +3794,7 @@ async function loadUsers() {
                 <td>${escapeHtml(u.username)}${isSelf ? ' <small style="color:#3498db;">(Sie)</small>' : ''}</td>
                 <td>${escapeHtml(u.displayName || '')}</td>
                 <td>${u.isAdmin ? '<span style="color:#27ae60;">✓</span>' : ''}</td>
-                <td>${escapeHtml(u.createdAt || '')}</td>
+                <td>${escapeHtml(formatDateTime(u.createdAt))}</td>
                 <td>
                     <button data-user-action="edit" data-uuid="${escapeHtml(u.uuid)}" class="btn-success" style="margin-right:5px;">Bearbeiten</button>
                     <button data-user-action="delete" data-uuid="${escapeHtml(u.uuid)}" data-username="${escapeHtml(u.username)}" class="btn-danger" ${isSelf ? 'disabled title="Eigener Account kann nicht gelöscht werden"' : ''}>Löschen</button>