fix(oauth): correct OAuth implementation to match bunq documentation

ts/bunq.classes.httpclient.ts

@@ -10,20 +10,12 @@ export class BunqHttpClient {
   private crypto: BunqCrypto;
   private context: IBunqApiContext;
   private requestCounter: number = 0;
-  private isOAuthMode: boolean = false;
 
   constructor(crypto: BunqCrypto, context: IBunqApiContext) {
     this.crypto = crypto;
     this.context = context;
   }
 
-  /**
-   * Set OAuth mode
-   */
-  public setOAuthMode(isOAuth: boolean): void {
-    this.isOAuthMode = isOAuth;
-  }
-
   /**
    * Update the API context (used after getting session token)
    */
@@ -44,20 +36,13 @@ export class BunqHttpClient {
     const body = options.body ? JSON.stringify(options.body) : undefined;
     
     // Add signature if required
-    // Skip signing for OAuth tokens or if explicitly disabled
-    if (options.useSigning !== false && !this.isOAuthMode) {
-      try {
-        const privateKey = this.crypto.getPrivateKey();
-        headers['X-Bunq-Client-Signature'] = this.crypto.createSignatureHeader(
-          options.method,
-          options.endpoint,
-          headers,
-          body || ''
-        );
-      } catch (error) {
-        // If no private key is available (e.g., OAuth mode), skip signing
-        // This is expected for OAuth tokens
-      }
+    if (options.useSigning !== false && this.crypto.getPrivateKey()) {
+      headers['X-Bunq-Client-Signature'] = this.crypto.createSignatureHeader(
+        options.method,
+        options.endpoint,
+        headers,
+        body || ''
+      );
     }
 
     // Make the request
@@ -81,8 +66,7 @@ export class BunqHttpClient {
       const response = await plugins.smartrequest.request(url, requestOptions);
       
       // Verify response signature if we have server public key
-      // Skip verification for OAuth tokens as they don't have installation keys
-      if (this.context.serverPublicKey && !this.isOAuthMode) {
+      if (this.context.serverPublicKey) {
         // Convert headers to string-only format
         const stringHeaders: { [key: string]: string } = {};
         for (const [key, value] of Object.entries(response.headers)) {