feat: add nano-banana-2 model type and 512px resolution

Add "nano-banana-2" to ModelType union for Gemini 3.1 Flash Image Preview.
Add "512" resolution tier. Add MODEL_DISPLAY_NAMES lookup to replace ternaries.

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

src/types/models.ts

@@ -18,8 +18,15 @@ export type AspectRatio =
   | "16:9"
   | "21:9";
 
-// Resolution Options (only supported by Nano Banana Pro)
-export type Resolution = "1K" | "2K" | "4K";
+// Resolution Options (supported by Nano Banana Pro and Nano Banana 2)
+export type Resolution = "512" | "1K" | "2K" | "4K";
 
 // Image Generation Model Options
-export type ModelType = "nano-banana" | "nano-banana-pro";
+export type ModelType = "nano-banana" | "nano-banana-pro" | "nano-banana-2";
+
+// Display names for image generation models
+export const MODEL_DISPLAY_NAMES: Record<ModelType, string> = {
+  "nano-banana": "Nano Banana",
+  "nano-banana-pro": "Nano Banana Pro",
+  "nano-banana-2": "Nano Banana 2",
+};