Plugins are switched on and off in a Plugins tab, the server keeps the state in data/plugins/plugins.json, and switched-off plugins are never delivered (#114)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/plugins/pluginApi.ts

@@ -18,6 +18,32 @@ export interface PluginHost {
   /** Whether a tab of this id exists already, built in or added by a plugin. */
   hasTab(id: string): boolean;
   addTab(id: string, label: string, content: Widget): void;
+  /** Takes a plugin's tab away again, its content with it. */
+  removeTab(id: string): void;
+}
+
+/**
+ * The host as one plugin sees it: every listener it subscribes and every tab it adds is written
+ * into `scope`, so switching the plugin off can take them away. An unsubscribe the plugin calls
+ * itself stays harmless when the scope calls it again.
+ */
+export function scopedHost(host: PluginHost, scope: { add(cleanup: () => void): void }): PluginHost {
+  return {
+    ...host,
+    onProjectChange(listener) {
+      let off: (() => void) | null = host.onProjectChange(listener);
+      const unsubscribe = () => {
+        off?.();
+        off = null;
+      };
+      scope.add(unsubscribe);
+      return unsubscribe;
+    },
+    addTab(id, label, content) {
+      host.addTab(id, label, content);
+      scope.add(() => host.removeTab(id));
+    },
+  };
 }
 
 /** A change of the asset list, as `project.on('change')` sees it: ids, names and types in order. */