feat(admin): allow editing boxes and deleting individual files

Introduce new admin capabilities to manage uploaded boxes and files:
- Add routes and handlers for editing boxes and deleting individual files.
- Implement `RemoveFileFromBox` in `UploadService` to delete a file's stored objects and remove it from the box (deleting the box if empty).
- Implement `AdminUpdateBox` in `UploadService` to update expiry, download limits, and clear password protection.
- Remove the unused `AdminFiles` handler.
- Add `.claude` to `.gitignore`.

backend/libs/handlers/admin.go

@@ -267,35 +267,6 @@ func (a *App) AdminDashboard(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
-func (a *App) AdminFiles(w http.ResponseWriter, r *http.Request) {
-	if !a.requireAdmin(w, r) {
-		return
-	}
-
-	stats, err := a.uploadService.AdminStats()
-	if err != nil {
-		http.Error(w, "unable to load admin stats", http.StatusInternalServerError)
-		return
-	}
-	boxes, err := a.adminBoxes(100)
-	if err != nil {
-		http.Error(w, "unable to load boxes", http.StatusInternalServerError)
-		return
-	}
-
-	a.renderPage(w, r, http.StatusOK, "admin.html", web.PageData{
-		Title:       "Admin files",
-		Description: "Manage Warpbox uploads.",
-		CurrentUser: a.currentPublicUser(r),
-		Data: adminPageData{
-			Stats:     stats,
-			Boxes:     boxes,
-			Section:   "files",
-			PageTitle: "Admin files",
-		},
-	})
-}
-
 func (a *App) AdminUsers(w http.ResponseWriter, r *http.Request) {
 	if !a.requireAdmin(w, r) {
 		return