Add brain emoji model selector with fullscreen modal

- Create ModelSelectionModal component with list-style interface
- Add brain emoji (🧠) indicator below chat input, left-aligned
- Show current model name in lowercase or red X if not configured
- Fullscreen modal displays all available models with configuration status
- Green checkmark for configured models, red alert for unconfigured
- Integrate with existing API key management and model selection logic
- Modal handles API key setup workflow when selecting unconfigured models
- Clean separation between compact indicator and full selection interface

examples/chat-ui/src/components/ChatApp.tsx

@@ -140,6 +140,8 @@ const ChatApp: React.FC<ChatAppProps> = () => {
             db={db as IDBPDatabase}
             selectedModel={selectedModel}
             onApiKeyUpdate={handleApiKeyUpdate}
+            onModelChange={handleModelChange}
+            apiKeyUpdateTrigger={apiKeyUpdateTrigger}
             mcpTools={mcpTools}
           />
         </div>