fix(update): add staging protection to switch_channel

switch_channel has three destructive code paths (git reset + clean,
git init + checkout --force, and rmtree + ZIP replace) that were
missing the _stage_preserved_items / _restore_preserved_items safety
net already applied to perform_update.

Wrap the channel-specific logic in a try/finally so preserved user
data (settings.json, civitai/, cache/, etc.) is physically moved
outside plugin_root before any git operation and always restored.

py/routes/update_routes.py

@@ -313,8 +313,10 @@ class UpdateRoutes:
             if os.path.exists(settings_path):
                 with open(settings_path, 'r', encoding='utf-8') as f:
                     settings_backup = f.read()
-                logger.info("Backed up settings.json before channel switch")
+                logger.info("Backed up settings.json before channel switch (%d bytes)", len(settings_backup))
 
+            staged_backup_dir, staged_items = _stage_preserved_items(plugin_root)
+            try:
                 git_folder = os.path.join(plugin_root, '.git')
 
                 if channel == 'nightly':
@@ -349,11 +351,13 @@ class UpdateRoutes:
                         success, new_version = await UpdateRoutes._download_and_replace_zip(plugin_root)
                     finally:
                         UpdateRoutes._restore_git(git_backup, git_folder, success, 'release')
+            finally:
+                _restore_preserved_items(plugin_root, staged_backup_dir, staged_items)
 
             if settings_backup and success:
                 with open(settings_path, 'w', encoding='utf-8') as f:
                     f.write(settings_backup)
-                logger.info("Restored settings.json after channel switch")
+                logger.info("Restored settings.json content after channel switch")
 
             if success:
                 return web.json_response({