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>

apps/web/src/components/admin/SystemSettingsClient.tsx

@@ -1181,13 +1181,15 @@ export function SystemSettingsClient() {
                     Model <InfoTooltip content="Gemini model for image generation. The default model supports image output." />
                   </span>
                 </label>
-                <input
-                  type="text"
+                <select
                   className={INPUT_CLASS}
-                  value={geminiModel}
+                  value={geminiModel || "gemini-2.5-flash-image"}
                   onChange={(e) => setGeminiModel(e.target.value)}
-                  placeholder="gemini-2.0-flash-preview-image-generation"
-                />
+                >
+                  <option value="gemini-2.5-flash-image">Gemini 2.5 Flash Image — fast, high-volume</option>
+                  <option value="gemini-3-pro-image-preview">Gemini 3 Pro Image Preview — high-fidelity</option>
+                  <option value="gemini-3.1-flash-image-preview">Gemini 3.1 Flash Image Preview — latest</option>
+                </select>
               </div>
             </div>
           </div>