🧹 Centralize SimpleEventEmitter class

Extract duplicated SimpleEventEmitter class from multiple service files into a shared utility file `js/utils/eventEmitter.js`.

- Created `js/utils/eventEmitter.js` with unified implementation.
- Refactored `AuthService`, `NostrService`, `ModerationService`, and `PlaybackService` to use the shared class.
- Added support for constructor-defined context strings for improved error logging.
- Added comprehensive unit tests for `SimpleEventEmitter`.
- Verified changes with targeted service tests.

Co-authored-by: PR0M3TH3AN <[email protected]>

js/utils/eventEmitter.js

@@ -0,0 +1,65 @@
+/**
+ * A lightweight event emitter used by services to communicate state changes
+ * and other events to the UI and other parts of the application.
+ */
+export class SimpleEventEmitter {
+  /**
+   * @param {Function|null} logger - Optional logger function for error reporting.
+   * @param {string} context - Optional context string for error messages.
+   */
+  constructor(logger = null, context = "SimpleEventEmitter") {
+    this.logger = typeof logger === "function" ? logger : null;
+    this.context = typeof context === "string" ? context : "SimpleEventEmitter";
+    this.listeners = new Map();
+  }
+
+  /**
+   * Registers a handler for a specific event.
+   * @param {string} eventName
+   * @param {Function} handler
+   * @returns {Function} Unsubscribe function.
+   */
+  on(eventName, handler) {
+    if (typeof handler !== "function") {
+      return () => {};
+    }
+    if (!this.listeners.has(eventName)) {
+      this.listeners.set(eventName, new Set());
+    }
+    const handlers = this.listeners.get(eventName);
+    handlers.add(handler);
+    return () => {
+      handlers.delete(handler);
+      if (!handlers.size) {
+        this.listeners.delete(eventName);
+      }
+    };
+  }
+
+  /**
+   * Emits an event to all registered handlers.
+   * @param {string} eventName
+   * @param {any} detail
+   */
+  emit(eventName, detail) {
+    const handlers = this.listeners.get(eventName);
+    if (!handlers || !handlers.size) {
+      return;
+    }
+
+    for (const handler of Array.from(handlers)) {
+      try {
+        handler(detail);
+      } catch (error) {
+        if (this.logger) {
+          try {
+            this.logger(`${this.context} listener for "${eventName}" threw`, error);
+          } catch (logError) {
+            // Fallback if the provided logger itself throws
+            console.warn(`[${this.context}] listener logger threw`, logError);
+          }
+        }
+      }
+    }
+  }
+}