Remove user rename feature; add www-data hint in WebDAV edit modal

- Removed PATCH /api/users/<name> route (rename_user)
- Removed renameModal from index.html
- Removed openRenameModal/submitRename from users.js
- Removed Rename button from users table
- Removed rename docs from README
- Added green info box in WebDAV edit modal: set group=www-data, mode=775

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -111,8 +111,6 @@ All routes require a login session. On first run you are redirected to `/setup`
 
 Users created in MicroNAS are standard Linux system accounts (`useradd -M -s /usr/sbin/nologin`). Their password is set in both the Linux shadow file (via `smbpasswd`) and the WebDAV htpasswd file automatically.
 
-**Rename** — the Rename button in the Users tab renames the Linux account (`usermod -l`). Because SMB and WebDAV store passwords by username, those entries are removed and must be reset after a rename. The UI shows a warning prompt.
-
 **Delete** — removes the Linux account, SMB entry, and WebDAV htpasswd entry.
 
 **Password change** — updates SMB and WebDAV passwords in sync. Changing a user's password also re-registers them in WebDAV if they were created before WebDAV was installed.