fix: use new v1beta/interactions endpoint for gemini 3.1 image generation instead of legacy predict

backend/server.js

@@ -380,8 +380,8 @@ app.post('/api/sparkplug/generate-images', authenticate, async (req, res) => {
     const apiKey = keys.NANO_BANANA_API_KEY || keys.GEMINI_API_KEY;
     if (!apiKey) return res.status(400).json({ error: 'Nano Banana (Gemini) API key missing' });
 
-    // We will use axios to call the Gemini Imagen REST API
-    const baseUrl = 'https://generativelanguage.googleapis.com/v1beta/models/gemini-3.1-flash-lite-image:predict';
+    // We will use axios to call the new Gemini 3.1 REST API via /interactions
+    const baseUrl = 'https://generativelanguage.googleapis.com/v1beta/interactions';
     
     const views = ['Front view', 'Back view', 'Left side view', 'Right side view'];
     const imageUrls = [];
@@ -391,22 +391,33 @@ app.post('/api/sparkplug/generate-images', authenticate, async (req, res) => {
       const fullPrompt = `${prompt}. ${view}, isolated on a pure white background, studio lighting.`;
       
       const payload = {
-        instances: [
-          {
-            prompt: fullPrompt
-          }
-        ],
-        parameters: {
-          sampleCount: 1,
-          outputOptions: { mimeType: 'image/png' }
-        }
+        model: "gemini-3.1-flash-lite-image",
+        input: [
+          { type: "text", text: fullPrompt }
+        ]
       };
 
       const response = await axios.post(`${baseUrl}?key=${apiKey}`, payload, {
         headers: { 'Content-Type': 'application/json' }
       });
 
-      const base64Image = response.data.predictions[0].bytesBase64Encoded;
+      // The new interactions endpoint returns the interaction object.
+      // We look for the image output.
+      let base64Image = null;
+      if (response.data.interaction?.output_image?.data) {
+        base64Image = response.data.interaction.output_image.data;
+      } else if (response.data.interaction?.output?.blocks) {
+        // Fallback for raw REST shape if output_image is an SDK-only convenience wrapper
+        const imgBlock = response.data.interaction.output.blocks.find(b => b.type === 'image' || b.image);
+        if (imgBlock) {
+          base64Image = imgBlock.image?.data || imgBlock.data;
+        }
+      }
+
+      if (!base64Image) {
+        throw new Error("Could not extract base64 image data from Gemini response: " + JSON.stringify(response.data));
+      }
+
       const buffer = Buffer.from(base64Image, 'base64');
       
       const filename = `sparkplug-${req.user.id}-${Date.now()}-${index}.png`;