feat: support folder uploads and sanitize upload paths

- Implement `cleanUploadDisplayName` in the backend to safely sanitize uploaded file paths, preserving directory structures while stripping unsafe characters and preventing path traversal.
- Add folder upload capability in the frontend using the Directory Picker API.
- Implement desktop notifications for completed uploads.

backend/static/js/45-preview.js

@@ -66,6 +66,7 @@
   bindLargeGate();
   bindThemeChanges();
   bindRenderFullscreen();
+  configureMediaSession();
   renderTabs();
   selectMode(state.defaultMode);
 
@@ -301,6 +302,32 @@
     document.addEventListener("fullscreenchange", updateRenderFullscreenButton);
   }
 
+  function configureMediaSession() {
+    if (!("mediaSession" in navigator) || typeof window.MediaMetadata !== "function") {
+      return;
+    }
+    if (!fileType.isAudio && !fileType.isVideo) {
+      return;
+    }
+    var artworkURL = "";
+    if (fileType.isVideo && els.videoPane) {
+      artworkURL = els.videoPane.getAttribute("poster") || state.iconURL || "";
+    } else {
+      artworkURL = state.iconURL || "";
+    }
+    var metadata = {
+      title: state.fileName || "Warpbox media",
+      artist: "Warpbox",
+      album: state.sizeLabel || state.contentType || ""
+    };
+    if (artworkURL) {
+      metadata.artwork = [
+        { src: window.Warpbox.absoluteURL(artworkURL), sizes: "512x512", type: "image/png" }
+      ];
+    }
+    navigator.mediaSession.metadata = new MediaMetadata(metadata);
+  }
+
   function ensureTextLoaded() {
     if (state.textLoaded) {
       return Promise.resolve(state.textSource);