feat: Ordner erstellen/löschen im File Explorer

Backend (WorkspaceEndpoint.cs):
- POST /api/workspace/mkdir – Ordner im Session-Workspace erstellen
- DELETE /api/workspace/dir – Leeren Ordner im Session-Workspace löschen
- POST /api/workspace/agent/mkdir – Ordner im Agent-Memory erstellen
- DELETE /api/workspace/agent/dir – Leeren Ordner im Agent-Memory löschen
- POST /api/workspace/browse/mkdir – Ordner im Folder-Browser erstellen
- Sicherheit: Path-Traversal-Schutz, Root nicht löschbar, nicht-leer → 409

Frontend (file-explorer.js):
- 📁+ Button im Header für neuen Ordner (File Explorer + Folder Browser)
- 🗑️ Button bei jedem Ordner (erscheint on hover, nur File Explorer)
- Validierung: ungültige Zeichen werden abgelehnt

CSS:
- base.css: Icon-Button mit zwei Icons (folder + plus)
- file-explorer.css: Ordner-Zeile als Flex (Label + Delete-Button)

wwwroot/js/file-explorer.js

@@ -37,6 +37,7 @@ function openFileExplorer(opts = {}) {
         <button class="fe-btn icon-btn" id="feBack" title="Zurück" disabled><i class="fa fa-arrow-left"></i></button>
         <span class="fe-path" id="fePath">/</span>
       </div>
+      <button class="fe-btn icon-btn" id="feNewDir" title="Neuer Ordner"><i class="fa fa-folder-open"></i> <i class="fa fa-plus"></i></button>
       <button class="fe-btn icon-btn" id="feClose"><i class="fa fa-times"></i></button>
     </div>
     <div class="fe-body">
@@ -60,6 +61,7 @@ function openFileExplorer(opts = {}) {
   document.body.appendChild(overlay);
 
   document.getElementById('feBack').addEventListener('click', feGoBack);
+  document.getElementById('feNewDir').addEventListener('click', feCreateDir);
   document.getElementById('feClose').addEventListener('click', closeFileExplorer);
   document.getElementById('feSelectBtn').addEventListener('click', feDoSelect);
   document.getElementById('feSaveBtn').addEventListener('click', feDoSave);
@@ -138,11 +140,24 @@ async function feLoadDir(path) {
 
       if (entry.type === 'dir') {
         item.classList.add('fe-dir');
-        item.innerHTML = '<i class="fa fa-folder"></i> ' + esc(entry.name);
-        item.addEventListener('click', () => {
+        const label = document.createElement('span');
+        label.className = 'fe-dir-label';
+        label.innerHTML = '<i class="fa fa-folder"></i> ' + esc(entry.name);
+        label.addEventListener('click', () => {
           _feState.history.push(path);
           feLoadDir(entry.path);
         });
+        item.appendChild(label);
+
+        const delBtn = document.createElement('button');
+        delBtn.className = 'fe-btn fe-dir-del';
+        delBtn.title = 'Ordner löschen (nur wenn leer)';
+        delBtn.innerHTML = '<i class="fa fa-trash-o"></i>';
+        delBtn.addEventListener('click', (e) => {
+          e.stopPropagation();
+          feDeleteDir(entry.path, entry.name);
+        });
+        item.appendChild(delBtn);
       } else {
         item.classList.add('fe-file');
         const icon = entry.isText ? '<i class="fa fa-file-text-o"></i>' : getFileIcon(entry.ext);
@@ -505,6 +520,7 @@ function openFolderBrowser(opts = {}) {
         <button class="fe-btn icon-btn" id="feBack" title="Zurück" disabled><i class="fa fa-arrow-left"></i></button>
         <span class="fe-path" id="fePath">/</span>
       </div>
+      <button class="fe-btn icon-btn" id="feNewDir" title="Neuer Ordner"><i class="fa fa-folder-open"></i> <i class="fa fa-plus"></i></button>
       <button class="fe-btn icon-btn" id="feClose"><i class="fa fa-times"></i></button>
     </div>
     <div class="fe-body">
@@ -522,6 +538,7 @@ function openFolderBrowser(opts = {}) {
   document.body.appendChild(overlay);
 
   document.getElementById('feBack').addEventListener('click', fbGoBack);
+  document.getElementById('feNewDir').addEventListener('click', fbCreateDir);
   document.getElementById('feClose').addEventListener('click', closeFileExplorer);
   document.getElementById('feSelectBtn').addEventListener('click', () => {
     if (_feState && _feState.onSelect) _feState.onSelect(_feState.currentAbsPath);
@@ -587,6 +604,83 @@ function fbGoBack() {
   fbLoadDir(prev);
 }
 
+// ── Folder Browser: Create Directory ──
+
+async function fbCreateDir() {
+  if (!_feState) return;
+
+  const name = prompt('Ordnername:');
+  if (!name || !name.trim()) return;
+
+  const dirName = name.trim();
+  if (/[\/\\:*?"<>|]/.test(dirName)) {
+    alert('Ungültiger Ordnername');
+    return;
+  }
+
+  const absPath = _feState.currentAbsPath.replace(/\/$/, '') + '/' + dirName;
+
+  try {
+    const resp = await fetch(`/api/workspace/browse/mkdir?path=${encodeURIComponent(absPath)}`, { method: 'POST' });
+    if (!resp.ok) {
+      const err = await resp.json().catch(() => ({}));
+      alert(err.error || 'Fehler beim Erstellen');
+      return;
+    }
+    fbLoadDir(_feState.currentAbsPath);
+  } catch (e) {
+    alert('Netzwerk-Fehler');
+  }
+}
+
+// ── File Explorer: Create / Delete Directory ──
+
+async function feCreateDir() {
+  if (!_feState) return;
+
+  const name = prompt('Ordnername:');
+  if (!name || !name.trim()) return;
+
+  const dirName = name.trim();
+  if (/[\/\\:*?"<>|]/.test(dirName)) {
+    alert('Ungültiger Ordnername');
+    return;
+  }
+
+  const newPath = _feState.currentPath ? _feState.currentPath + '/' + dirName : dirName;
+
+  try {
+    const url = _feApiUrl('mkdir', `path=${encodeURIComponent(newPath)}`);
+    const resp = await fetch(url, { method: 'POST' });
+    if (!resp.ok) {
+      const err = await resp.json().catch(() => ({}));
+      alert(err.error || 'Fehler beim Erstellen');
+      return;
+    }
+    feLoadDir(_feState.currentPath);
+  } catch (e) {
+    alert('Netzwerk-Fehler');
+  }
+}
+
+async function feDeleteDir(dirPath, dirName) {
+  if (!_feState) return;
+  if (!confirm(`Ordner "${dirName}" löschen?\n(Nur möglich wenn leer)`)) return;
+
+  try {
+    const url = _feApiUrl('dir', `path=${encodeURIComponent(dirPath)}`);
+    const resp = await fetch(url, { method: 'DELETE' });
+    if (!resp.ok) {
+      const err = await resp.json().catch(() => ({}));
+      alert(err.error || 'Fehler beim Löschen');
+      return;
+    }
+    feLoadDir(_feState.currentPath);
+  } catch (e) {
+    alert('Netzwerk-Fehler');
+  }
+}
+
 // ── Helpers ──
 
 function getFileIcon(ext) {