activeUsers (#4936)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/core/services/apiClientSetup.ts

@@ -1,5 +1,14 @@
 import { AxiosInstance } from 'axios';
+import { getBrowserId } from '@app/utils/browserIdentifier';
 
-export function setupApiInterceptors(_client: AxiosInstance): void {
-  // Core version: no interceptors to add
+export function setupApiInterceptors(client: AxiosInstance): void {
+  // Add browser ID header for WAU tracking
+  client.interceptors.request.use(
+    (config) => {
+      const browserId = getBrowserId();
+      config.headers['X-Browser-Id'] = browserId;
+      return config;
+    },
+    (error) => Promise.reject(error)
+  );
 }

frontend/src/core/utils/browserIdentifier.ts

@@ -0,0 +1,46 @@
+/**
+ * Browser identifier utility for anonymous usage tracking
+ * Generates and persists a unique UUID in localStorage for WAU tracking
+ */
+
+const BROWSER_ID_KEY = 'stirling_browser_id';
+
+/**
+ * Gets or creates a unique browser identifier
+ * Used for Weekly Active Users (WAU) tracking in no-login mode
+ */
+export function getBrowserId(): string {
+  try {
+    // Try to get existing ID from localStorage
+    let browserId = localStorage.getItem(BROWSER_ID_KEY);
+
+    if (!browserId) {
+      // Generate new UUID v4
+      browserId = generateUUID();
+      localStorage.setItem(BROWSER_ID_KEY, browserId);
+    }
+
+    return browserId;
+  } catch (error) {
+    // Fallback to session-based ID if localStorage is unavailable
+    console.warn('localStorage unavailable, using session-based ID', error);
+    return `session_${generateUUID()}`;
+  }
+}
+
+/**
+ * Generates a UUID v4
+ */
+function generateUUID(): string {
+  // Use crypto.randomUUID if available (modern browsers)
+  if (typeof crypto !== 'undefined' && crypto.randomUUID) {
+    return crypto.randomUUID();
+  }
+
+  // Fallback to manual UUID generation
+  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, (c) => {
+    const r = (Math.random() * 16) | 0;
+    const v = c === 'x' ? r : (r & 0x3) | 0x8;
+    return v.toString(16);
+  });
+}