fix: correct Gemini model names + add model dropdown

Corrected model names (per Google AI docs):
- gemini-2.5-flash-image (was gemini-2.0-flash-preview-image-generation)
- gemini-3-pro-image-preview (Nano Banana Pro)
- gemini-3.1-flash-image-preview (Nano Banana 2)

UI: replaced text input with dropdown selector showing all 3 models
with human-readable descriptions.

Default changed to gemini-2.5-flash-image (fast, high-volume).

AI Assistant: generate_project_cover tool description updated to be
provider-agnostic (works with both DALL-E and Gemini).

Co-Authored-By: claude-flow <ruv@ruv.net>

packages/api/src/gemini-client.ts

@@ -15,7 +15,7 @@ export function isGeminiConfigured(settings: GeminiSettings | null | undefined):
 export async function generateGeminiImage(
   apiKey: string,
   prompt: string,
-  model = "gemini-2.0-flash-preview-image-generation",
+  model = "gemini-2.5-flash-image",
 ): Promise<string> {
   const fullPrompt = `Generate a professional, cinematic cover image for a 3D production project. ${prompt}`;