fix: update nano-banana model ID to stable gemini-2.5-flash-image

The old preview model ID (gemini-2.5-flash-preview-image-generation) was
deprecated. Updated to the current stable ID across provider, cost
calculator, tests, and docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -65,7 +65,7 @@ All application state lives in `workflowStore.ts` using Zustand. Key patterns:
 ## AI Models
 
 Image generation models (these exist and are recently released):
-- `gemini-2.5-flash-preview-image-generation` → internal name: `nano-banana`
+- `gemini-2.5-flash-image` → internal name: `nano-banana`
 - `gemini-3-pro-image-preview` → internal name: `nano-banana-pro`
 
 LLM models:

src/app/api/generate/__tests__/route.test.ts

@@ -525,7 +525,7 @@ describe("/api/generate route", () => {
       expect(data.success).toBe(true);
       expect(mockGenerateContent).toHaveBeenCalledWith(
         expect.objectContaining({
-          model: "gemini-2.5-flash-preview-image-generation",
+          model: "gemini-2.5-flash-image",
         })
       );
     });

src/app/api/generate/providers/gemini.ts

@@ -12,7 +12,7 @@ import { GenerateResponse, ModelType } from "@/types";
  * Map model types to Gemini model IDs
  */
 export const MODEL_MAP: Record<ModelType, string> = {
-  "nano-banana": "gemini-2.5-flash-preview-image-generation",
+  "nano-banana": "gemini-2.5-flash-image",
   "nano-banana-pro": "gemini-3-pro-image-preview",
 };
 

src/utils/costCalculator.ts

@@ -146,7 +146,7 @@ export function calculatePredictedCost(
 
     // Fallback to hardcoded Gemini pricing for legacy models
     if (provider === "gemini") {
-      if (modelId === "nano-banana" || modelId === "gemini-2.5-flash-preview-image-generation") {
+      if (modelId === "nano-banana" || modelId === "gemini-2.5-flash-image") {
         return { unitCost: PRICING["nano-banana"]["1K"], unit: "image" };
       }
       if (modelId === "nano-banana-pro" || modelId === "gemini-3-pro-image-preview") {