"nosub" volflag circumventable to create subdirectories

### Describe the bug
"nosub" volflag does not work as intended when uploading directories that contain files

### To Reproduce
In a share with the "nosub" volflag (cannot create subfolders), upload a directory (or nested directories) that contain at least one file. Upon receiving error "server rejected the request to initiate upload; error 500: no subdirectories allowed", any directories that contained (recursive OK) a file will have been created anyway, empty.

### Expected behavior
No folder is created, as no subfolders are allowed to be created (mkdir UI feature does not work)

### Screenshots

### Server details
- Oracle Linux Server 9.8 aarch64 / Linux 6.12.0-205.92.4.2.el9uek.aarch64 / KVM Virtual Machine (virt-7.2)
- copyparty v1.20.21 "sftp is fine too" (2026-08-17)
CPython v3.9.25 on Linux64 [GCC 11.5.0 20240719 (Red Hat 11.5.0-14.0.1)]
sqlite 3.34.1*1 | jinja 2.11.3 | pyftpd 1.5.10 | tftp 0.4.0 | miko (None)
- systemd-service
- global config:
```
[global]
daw
e2d
e2ts
-grid
e2dsa
magic
favico:
theme: 7
th-ff-jpg
rproxy: 1
no-hash: .
i: 127.0.0.1
name: Storage |
xff-src: 127.0.0.1
hist: ~/.cache/copyparty
xff-hdr: cf-connecting-ip
lo: $LOGS_DIRECTORY/%m-%d-%Y.log
html-head:
```
- folder config:
```
[/]
/files
accs:
rwmd.a: @admin
rwm: @oomfs
r: *
flags:
grid
e2ds
ufavico: 🐱
ui_nosrvi
ui_nolbar
ui_noctxb
ui_norepl
sz: 0-67m
vmaxn: 256
maxn: 15,600
nosub
```
- directory persmissions:
`drwxrwsr-x+ 4 copyparty worker 4096 Aug 19 02:34 /files`
- access control list:
```
# file: files
# owner: copyparty
# group: worker
# flags: -s-
user::rwx
group::rwx
group:worker:rwx
mask::rwx
other::r-x
default:user::rwx
default:group::rwx
default:group:worker:rwx
default:mask::rwx
default:other::r-x
```
- nginx reverse proxy on OCI run through cloudflare

### Client details
The server shouldn't allowed a directory to be created so this shouldn't matter but for the sake of thoroughness:
* the device type and model: PC
* OS version: Windows 10 Home 22H2 19045.7663
* browser version: Zen 1.21.14b (64-bit)

### Server log
```
02:48:56.059 ***.**.***.*** 45624 LOG: GET / @oomf
02:48:56.439 ***.**.***.*** 45624 LOG: GET /?tree= @oomf
02:49:04.803 ***.**.***.*** 45668 LOG: GET / @oomf
02:49:04.991 ***.**.***.*** 45668 LOG: GET /?tree= @oomf
02:49:48.100 ***.**.***.*** 45668 LOG: POST /bug_test/bug_test_2/bug_test_3/ @oomf
02:49:48.101 ***.**.***.*** 45668 LOG: POST len=113 type=utf-8 ip=***.**.***.*** user=oomf req='/bug_test/bug_test_2/bug_test_3/' json={'name': 'a.txt', 'size': 17, 'lmod': 1787122180.379, 'life': 0, 'hash': '1 chunks'}
02:49:48.103 ***.**.***.*** 45668 WARN: http500: no subdirectories allowed, '/bug_test/bug_test_2/bug_test_3'
02:49:51.289 ***.**.***.*** 50366 LOG: GET / @oomf
02:49:51.451 ***.**.***.*** 50366 LOG: GET /place/copyparty.css @oomf
02:49:51.451 ***.**.***.*** 50366 LOG: /place/copyparty.css plain 304
02:49:51.548 ***.**.***.*** 50366 LOG: GET /?tree= @oomf
02:49:53.387 ***.**.***.*** 50366 LOG: GET /bug_test/?ls @oomf
02:49:54.116 ***.**.***.*** 50366 LOG: GET /bug_test/bug_test_2/?ls&rtt=50 @oomf
02:49:54.685 ***.**.***.*** 50366 LOG: GET /bug_test/bug_test_2/bug_test_3/?ls&rtt=49 @oomf
```