fix: OnlyOffice/Preview zeigt immer aktuelle Version (kein Cache)

Drei Cache-Ebenen gefixt:
- Vue Router: :key=fullPath erzwingt Komponenten-Neuaufbau bei
  jeder Navigation (kein Wiederverwenden alter Instanzen)
- Frontend: Cache-Bust Parameter an Preview + OnlyOffice API-Calls
- Backend: No-Cache Headers (Cache-Control, Pragma) auf Preview-Endpunkt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/AppLayout.vue

@@ -67,7 +67,7 @@
     </aside>
 
     <main class="main-content">
-      <router-view />
+      <router-view :key="$route.fullPath" />
     </main>
   </div>
 </template>

frontend/src/views/PreviewView.vue

@@ -104,6 +104,7 @@ const auth = useAuthStore()
 const toast = useToast()
 
 const fileId = route.params.fileId
+const cacheBust = Date.now()
 const fileName = ref('')
 const previewType = ref('')
 const previewUrl = ref('')
@@ -135,12 +136,12 @@ async function loadPreview() {
   loading.value = true
   try {
     // For Office files, try OnlyOffice first
-    const previewRes = await apiClient.get(`/files/${fileId}/preview`)
+    const previewRes = await apiClient.get(`/files/${fileId}/preview?_=${cacheBust}`)
     fileName.value = previewRes.data.name || ''
 
     if (isOfficeFile(fileName.value)) {
       try {
-        const ooRes = await apiClient.get(`/files/${fileId}/onlyoffice-config`)
+        const ooRes = await apiClient.get(`/files/${fileId}/onlyoffice-config?_=${cacheBust}`)
         if (ooRes.data.available) {
           onlyOfficeMode.value = true
           loading.value = false