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>

static/js/shares.js

@@ -259,6 +259,11 @@ async function openEditModal(type, share) {
                 </label>
                 <input type="text" id="editWebdavUsers" class="form-control" value="${esc(share.users || '')}">
             </div>
+            <div class="mb-3 px-2 py-2 rounded" style="background:#1a2a1a; border:1px solid #2d4a2d; font-size:.82rem; color:#8dba8d">
+                WebDAV writes files as <code style="color:#3fb950">www-data</code>.
+                Set the folder group to <code style="color:#3fb950">www-data</code> and mode to <code style="color:#3fb950">775</code> below,
+                or add <code style="color:#3fb950">www-data</code> to your share group.
+            </div>
             ${permSection}`;
     } else {
         const isRo = /\(ro[,)]/.test(share.clients);