feat(download): add dynamic OG metadata and fix thumbnail caching

- Register a new route for box Open Graph images (`/d/{boxID}/og-image.jpg`).
- Dynamically set the download page title, description, and OG image URL based on box state (e.g., file count, expiration, password protection).
- Introduce `servePlaceholderThumbnail` to serve fallback thumbnails with `Cache-Control: no-store, must-revalidate`. This ensures the browser requests the real thumbnail once it is generated instead of caching the placeholder.

backend/libs/handlers/download.go

@@ -76,9 +76,18 @@ func (a *App) DownloadPage(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	expiresLabel := box.ExpiresAt.Format("Jan 2, 2006 15:04 MST")
+	title := "Shared files on Warpbox"
+	description := fmt.Sprintf("%d file%s shared via Warpbox · expires %s", len(box.Files), plural(len(box.Files)), expiresLabel)
+	if locked && box.Obfuscate {
+		title = "Protected Warpbox link"
+		description = "This shared box is password protected."
+	}
+
 	a.renderPage(w, r, http.StatusOK, "download.html", web.PageData{
-		Title:       "Download files",
-		Description: "Download files shared through Warpbox.",
+		Title:       title,
+		Description: description,
+		ImageURL:    absoluteURL(r, fmt.Sprintf("/d/%s/og-image.jpg", box.ID)),
 		Data: downloadPageData{
 			Box:           boxView{ID: box.ID},
 			Files:         files,
@@ -87,11 +96,18 @@ func (a *App) DownloadPage(w http.ResponseWriter, r *http.Request) {
 			Obfuscated:    box.Obfuscate,
 			DownloadCount: box.DownloadCount,
 			MaxDownloads:  box.MaxDownloads,
-			ExpiresLabel:  box.ExpiresAt.Format("Jan 2, 2006 15:04 MST"),
+			ExpiresLabel:  expiresLabel,
 		},
 	})
 }
 
+func plural(n int) string {
+	if n == 1 {
+		return ""
+	}
+	return "s"
+}
+
 func (a *App) DownloadFile(w http.ResponseWriter, r *http.Request) {
 	box, file, ok := a.loadFileForRequest(w, r)
 	if !ok {
@@ -141,13 +157,18 @@ func (a *App) Thumbnail(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if a.uploadService.IsProtected(box) && box.Obfuscate && !a.isBoxUnlocked(r, box) {
-		http.ServeFile(w, r, filepath.Join(a.cfg.StaticDir, "img", "file-placeholder.webp"))
+		a.servePlaceholderThumbnail(w, r)
 		return
 	}
 
 	object, err := a.uploadService.OpenThumbnailObject(r.Context(), box, file)
 	if err != nil {
-		http.ServeFile(w, r, filepath.Join(a.cfg.StaticDir, "img", "file-placeholder.webp"))
+		// The thumbnail isn't generated yet (background job pending). Serve the
+		// placeholder but mark it non-cacheable, otherwise the browser would
+		// keep showing the placeholder until a hard refresh once the real
+		// thumbnail lands. The real thumbnail below is content-stable, so it
+		// gets a long immutable cache.
+		a.servePlaceholderThumbnail(w, r)
 		return
 	}
 	defer object.Body.Close()
@@ -156,6 +177,14 @@ func (a *App) Thumbnail(w http.ResponseWriter, r *http.Request) {
 	http.ServeContent(w, r, file.ID+"-thumbnail.jpg", object.ModTime, readSeekCloser(object.Body))
 }
 
+// servePlaceholderThumbnail serves the fallback image with no-store so the
+// browser re-requests on the next load and picks up the real thumbnail as soon
+// as it has been generated.
+func (a *App) servePlaceholderThumbnail(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Cache-Control", "no-store, must-revalidate")
+	http.ServeFile(w, r, filepath.Join(a.cfg.StaticDir, "img", "file-placeholder.webp"))
+}
+
 func (a *App) UnlockBox(w http.ResponseWriter, r *http.Request) {
 	box, err := a.uploadService.GetBox(r.PathValue("boxID"))
 	if err != nil {