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>

templates/index.html

@@ -102,27 +102,6 @@
     </div>
 </div>
 
-<!-- Rename user modal -->
-<div class="modal fade" id="renameModal" tabindex="-1">
-    <div class="modal-dialog modal-sm">
-        <div class="modal-content">
-            <div class="modal-header">
-                <h5 class="modal-title">Rename User</h5>
-                <button type="button" class="btn-close btn-close-white" data-bs-dismiss="modal"></button>
-            </div>
-            <div class="modal-body">
-                <input type="hidden" id="renameOldName">
-                <label class="form-label" style="color:var(--muted); font-size:.85rem">New Username</label>
-                <input type="text" id="renameNewName" class="form-control mb-2">
-                <div class="small" style="color:var(--yellow)">SMB and WebDAV password must be reset after rename.</div>
-            </div>
-            <div class="modal-footer">
-                <button type="button" class="btn btn-secondary btn-sm" data-bs-dismiss="modal">Cancel</button>
-                <button type="button" class="btn btn-primary btn-sm" onclick="submitRename()">Rename</button>
-            </div>
-        </div>
-    </div>
-</div>
 
 <!-- Header -->
 <div class="topbar d-flex justify-content-between align-items-center">