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/db/prisma/schema.prisma

@@ -1449,7 +1449,7 @@ model SystemSettings {
   azureDalleApiKey        String?  // Optional: separate API key for DALL-E
   // Gemini image generation
   geminiApiKey            String?
-  geminiModel             String?  @default("gemini-2.0-flash-preview-image-generation")
+  geminiModel             String?  @default("gemini-2.5-flash-image")
   imageProvider           String?  @default("dalle")  // "dalle" | "gemini"
   updatedAt               DateTime @updatedAt