feat(server): add ExtAuth logout URL configuration (#5074)

* feat(server): add ExtAuth logout URL configuration (#4467)

When external authentication (reverse proxy auth) is active, the Logout
button is hidden because authentication is managed externally. Many
external auth services (Authelia, Authentik, Keycloak) provide a logout
URL that can terminate the session.

Add `ExtAuth.LogoutURL` config option that, when set, shows the Logout
button in the UI and redirects the user to the external auth provider's
logout endpoint instead of the Navidrome login page.

* feat(server): add validation for ExtAuth logout URL configuration

* feat(server): refactor ExtAuth logout URL validation to a reusable function

* fix(configuration): rename URL validation functions for consistency

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(configuration): rename URL validation functions for consistency

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

server/serve_index.go

@@ -76,6 +76,7 @@ func serveIndex(ds model.DataStore, fs fs.FS, shareInfo *model.Share) http.Handl
 			"separator":                 string(os.PathSeparator),
 			"enableInspect":             conf.Server.Inspect.Enabled,
 			"pluginsEnabled":            conf.Server.Plugins.Enabled,
+			"extAuthLogoutURL":          conf.Server.ExtAuth.LogoutURL,
 		}
 		if strings.HasPrefix(conf.Server.UILoginBackgroundURL, "/") {
 			appConfig["loginBackgroundURL"] = path.Join(conf.Server.BasePath, conf.Server.UILoginBackgroundURL)