DMS redesign: 3-column explorer layout with toolbar, source tree, file details panel

- Add backend endpoints: GET /dms/files, GET /dms/folders/{id}/files
- New SourceTree component: sources (Meine Dateien, Geteilte Ordner, Externe Speicher)
- New FileExplorer component: 5 view modes (list, table, icons-sm/md/lg)
- New FileDetails component: right panel with file metadata and actions
- Rewrite Dms.tsx: PluginToolbar + 3-column ResizablePanel layout
- Update DmsFile type: size_bytes + uploaded_by (backward compat aliases)
- Add 60+ i18n keys for de/en

frontend/src/i18n/locales/en.json

@@ -653,16 +653,16 @@
   "dms": {
     "title": "Files",
     "trash": "Trash",
-    "newFolder": "New Folder",
+    "newFolder": "New folder",
     "upload": "Upload",
-    "search": "Search files",
+    "search": "Search",
     "searchPlaceholder": "Search...",
     "folders": "Folders",
     "files": "Files",
     "uploadDropHere": "Drag files here or click to select",
-    "shareWith": "Shares",
+    "shareWith": "Share",
     "addShare": "Add share",
-    "userOrGroup": "User or Group",
+    "userOrGroup": "User or group",
     "userId": "User ID",
     "groupId": "Group ID",
     "permissionLevel": "Permission level",
@@ -671,9 +671,58 @@
     "expiryDate": "Expiry date",
     "createShareLink": "Create link",
     "loading": "Loading",
-    "shared": "Share created",
+    "shared": "Shared",
     "linkCopied": "Link copied",
-    "uploadError": "Upload failed"
+    "uploadError": "Upload failed",
+    "sources": "Sources",
+    "myFiles": "My Files",
+    "sharedFolders": "Shared Folders",
+    "externalStorage": "External Storage",
+    "externalStorageHint": "External storage can be added in settings",
+    "viewList": "List view",
+    "viewTable": "Table view",
+    "viewIconsSm": "Small icons",
+    "viewIconsMd": "Medium icons",
+    "viewIconsLg": "Large icons",
+    "details": "Details",
+    "toggleDetails": "Toggle details",
+    "fileSize": "Size",
+    "fileType": "Type",
+    "fileModified": "Modified",
+    "fileCreated": "Created",
+    "uploadedBy": "Uploaded by",
+    "noFileSelected": "No file selected",
+    "noFileSelectedHint": "Select a file to see details",
+    "download": "Download",
+    "rename": "Rename",
+    "allFiles": "All Files",
+    "sortBy": "Sort by",
+    "sortName": "Name",
+    "sortSize": "Size",
+    "sortDate": "Date",
+    "sortType": "Type",
+    "bulkMove": "Move",
+    "bulkDelete": "Delete",
+    "selected": "selected",
+    "confirmDelete": "Are you sure you want to delete this file?",
+    "confirmBulkDelete": "Are you sure you want to delete the selected files?",
+    "createFolder": "Create folder",
+    "folderName": "Folder name",
+    "cancel": "Cancel",
+    "delete": "Delete",
+    "preview": "Preview",
+    "share": "Share",
+    "noFiles": "No files",
+    "uploadSuccess": "Upload successful",
+    "uploading": "Uploading",
+    "deleted": "Deleted",
+    "bulkSelect": "Select",
+    "bulkMoveTitle": "Move files",
+    "bulkDeleteTitle": "Delete files",
+    "selectTargetFolder": "Select target folder",
+    "icon": {
+      "other": "File"
+    }
   },
   "permissions": {
     "user": "User",