feat(mockup): add OpenAI Codex (gpt-5.6-sol + built-in image_gen) raster mockup generator

public/app.js

@@ -1803,7 +1803,7 @@ function bindGlobalEvents() {
     const editInstruction = document.getElementById("nb-edit-instruction-input").value.trim();
 
     btn.disabled = true;
-    statusLabel.textContent = "🍌 Calling Nano Banana 2.1 (gemini-nano-banana-2.1)...";
+    statusLabel.textContent = "🎨 Generating high-res AI mockup via OpenAI Codex (takes ~15–25s)...";
 
     try {
       const res = await apiFetch(`/api/ideas/${state.activeIdeaId}/mockups/nano-banana`, {

public/index.html

@@ -534,7 +534,7 @@
 
               <div class="studio-action-row">
                 <button type="button" id="btn-generate-nano-banana" class="btn btn-primary">
-                  🍌 Generate with Nano Banana 2.1
+                  🎨 Generate AI Mockup (Codex / Nano Banana)
                 </button>
                 <span id="nb-generation-status" class="muted-label"></span>
               </div>
@@ -684,9 +684,10 @@
           <small id="settings-key-status" class="muted-label"></small>
         </div>
         <div class="field-group">
-          <label for="settings-model-select">Nano Banana Model</label>
+          <label for="settings-model-select">AI Mockup Engine / Model</label>
           <select id="settings-model-select" class="filter-select full-width">
-            <option value="gemini-nano-banana-2.1">gemini-nano-banana-2.1 — Nano Banana 2.1 (Default High-Efficiency)</option>
+            <option value="codex-image-gen">codex-image-gen — OpenAI Codex (gpt-5.6-sol + Built-in image_gen) [Recommended]</option>
+            <option value="gemini-nano-banana-2.1">gemini-nano-banana-2.1 — Nano Banana 2.1 (Google Gemini)</option>
             <option value="gemini-3-pro-image">gemini-3-pro-image — Nano Banana Pro (High-Detail Studio)</option>
             <option value="gemini-3.1-flash-lite-image">gemini-3.1-flash-lite-image — Nano Banana 2 Lite (Ultra-Fast)</option>
           </select>