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/static/css/50-admin.css

@@ -106,6 +106,38 @@
   font-weight: 650;
 }
 
+.sort-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3rem;
+  color: var(--muted-foreground);
+  font-weight: 650;
+  text-decoration: none;
+}
+
+.sort-link:hover,
+.sort-link.is-sorted {
+  color: var(--foreground);
+}
+
+.sort-arrow {
+  font-size: 0.7rem;
+}
+
+.pagination {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.35rem;
+  margin-top: 1rem;
+}
+
+.pagination-summary {
+  margin: 0.6rem 0 0;
+  color: var(--muted-foreground);
+  font-size: 0.78rem;
+}
+
 .table-actions {
   display: flex;
   align-items: flex-start;