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/users.js

@@ -22,8 +22,6 @@ async function loadUsers() {
                 </span>
             </td>
             <td>
-                <button class="btn btn-sm me-1" style="border:1px solid var(--border); color:var(--muted)"
-                    onclick="openRenameModal(${nameJson})">Rename</button>
                 <button class="btn btn-sm me-1" style="border:1px solid var(--border); color:var(--muted)"
                     onclick="openPwModal(${nameJson})">Password</button>
                 <button class="btn btn-sm btn-danger"
@@ -97,30 +95,6 @@ async function submitPassword() {
     showToast(`Password updated for "${name}".`);
 }
 
-function openRenameModal(name) {
-    document.getElementById('renameOldName').value = name;
-    document.getElementById('renameNewName').value = name;
-    new bootstrap.Modal(document.getElementById('renameModal')).show();
-}
-
-async function submitRename() {
-    const oldName = document.getElementById('renameOldName').value;
-    const newName = document.getElementById('renameNewName').value.trim();
-    if (!newName) return showToast('New username is required.', 'danger');
-
-    const res = await apiFetch(`/api/users/${encodeURIComponent(oldName)}`, {
-        method: 'PATCH', headers: {'Content-Type': 'application/json'},
-        body: JSON.stringify({name: newName}),
-    });
-    if (!res) return;
-    const data = await res.json();
-    bootstrap.Modal.getInstance(document.getElementById('renameModal')).hide();
-    if (!res.ok) return showToast('Error: ' + (data.error || res.status), 'danger');
-    if (data.warning) showToast(data.warning, 'warning');
-    else showToast(`Renamed to "${newName}".`);
-    loadUsers();
-    loadGroups();
-}
 
 function openAdminSettings() {
     document.getElementById('adminCurrentPw').value = '';