Add Remove Background feature using AI (rembg)

Frontend:
- Added "Remove Background (AI)" to Image menu
- Created remove_background.js module with dialog options
- Added removeBackground method to API service

Backend:
- Added /tools/remove-background-base64 endpoint for miniPaint frontend
- Uses rembg library for AI-powered background removal

Features:
- Automatically detects main subject and removes background
- Option to create as new layer or replace current
- Enables transparency mode after removal
- Works with any image layer

frontend/src/js/services/api.js

@@ -71,6 +71,30 @@ class ApiService {
         return response.json();
     }
 
+    /**
+     * Remove background from image using AI (rembg)
+     * @param {string} imageData - Base64 encoded image data
+     * @returns {Promise<{result: string, width: number, height: number}>} - Base64 encoded result with transparency
+     */
+    async removeBackground(imageData) {
+        const response = await fetch(`${this.baseUrl}/tools/remove-background-base64`, {
+            method: 'POST',
+            headers: {
+                'Content-Type': 'application/json',
+            },
+            body: JSON.stringify({
+                image: imageData,
+            }),
+        });
+
+        if (!response.ok) {
+            const error = await response.json().catch(() => ({ detail: 'Unknown error' }));
+            throw new Error(error.detail || `Remove background request failed: ${response.status}`);
+        }
+
+        return response.json();
+    }
+
     /**
      * Health check for the backend
      * @returns {Promise<boolean>}