fix(oauth): fix OAuth token authentication flow for existing installations

ts/bunq.classes.apicontext.ts

@@ -171,31 +171,49 @@ export class BunqApiContext {
   }
 
   /**
-   * Initialize with existing OAuth session (skip installation/device/session creation)
+   * Initialize with existing installation and device (for OAuth tokens that already completed these steps)
    */
-  public async initWithExistingSession(): Promise<void> {
-    // For OAuth tokens that already have a session, we just need to:
-    // 1. Use the OAuth token as the session token
-    // 2. Set OAuth mode for proper expiry handling
+  public async initWithExistingInstallation(): Promise<void> {
+    // For OAuth tokens that already have installation/device but need a new session
+    // We need to:
+    // 1. Try to load existing installation/device info
+    // 2. Create a new session using the OAuth token as the secret
     
-    this.context.sessionToken = this.options.apiKey;
+    const existingContext = await this.loadContext();
     
-    // Create session instance with existing token
-    this.session = new BunqSession(this.crypto, this.context);
-    this.session.setOAuthMode(true);
-    
-    // Try to get user info to validate the session
-    try {
-      // This will test if the session is valid
-      const testClient = this.session.getHttpClient();
-      const response = await testClient.get('/v1/user');
+    if (existingContext && existingContext.clientPrivateKey && existingContext.clientPublicKey) {
+      // Restore crypto keys from previous installation
+      this.crypto.setKeys(
+        existingContext.clientPrivateKey,
+        existingContext.clientPublicKey
+      );
       
-      if (response && response.Response) {
-        console.log('Successfully reused existing OAuth session');
+      // Update context with existing installation data
+      this.context = { ...this.context, ...existingContext };
+      
+      // Create new session instance
+      this.session = new BunqSession(this.crypto, this.context);
+      
+      // Try to create a new session with the OAuth token
+      try {
+        await this.session.init(
+          this.options.deviceDescription,
+          this.options.permittedIps || [],
+          true // skipInstallationAndDevice = true
+        );
+        
+        if (this.options.isOAuthToken) {
+          this.session.setOAuthMode(true);
+        }
+        
         await this.saveContext();
+        console.log('Successfully created new session with existing installation');
+      } catch (error) {
+        throw new Error(`Failed to create session with OAuth token: ${error.message}`);
       }
-    } catch (error) {
-      throw new Error(`Failed to reuse OAuth session: ${error.message}`);
+    } else {
+      // No existing installation, fall back to full init
+      throw new Error('No existing installation found, full initialization required');
     }
   }
 }