Update default negative prompt and add image clear button

- Replace short negative prompt with comprehensive list
- Add X button to clear selected image before generating
- Allow selecting a new image by clearing the current one

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/public/js/app.js

@@ -277,6 +277,8 @@ document.querySelectorAll('.nav-link').forEach(link => {
 const imageInput = document.getElementById('image-input');
 const imageUploadArea = document.getElementById('image-upload-area');
 const previewImage = document.getElementById('preview-image');
+const previewContainer = document.getElementById('preview-container');
+const clearImageBtn = document.getElementById('clear-image-btn');
 
 imageInput.addEventListener('change', (e) => {
     const file = e.target.files[0];
@@ -284,7 +286,7 @@ imageInput.addEventListener('change', (e) => {
         const reader = new FileReader();
         reader.onload = (event) => {
             previewImage.src = event.target.result;
-            previewImage.classList.remove('hidden');
+            previewContainer.classList.remove('hidden');
             imageUploadArea.classList.add('has-file');
             base64Image = event.target.result.split(',')[1];
         };
@@ -292,6 +294,14 @@ imageInput.addEventListener('change', (e) => {
     }
 });
 
+clearImageBtn.addEventListener('click', () => {
+    previewImage.src = '';
+    previewContainer.classList.add('hidden');
+    imageUploadArea.classList.remove('has-file');
+    imageInput.value = '';
+    base64Image = '';
+});
+
 // Generation
 document.getElementById('generate-form').addEventListener('submit', async (e) => {
     e.preventDefault();