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/css/style.css

@@ -339,6 +339,12 @@ body {
     color: var(--gray-600);
 }
 
+.preview-container {
+    position: relative;
+    display: inline-block;
+    max-width: 100%;
+}
+
 .preview-image {
     max-width: 100%;
     max-height: 300px;
@@ -346,6 +352,28 @@ body {
     object-fit: contain;
 }
 
+.clear-image-btn {
+    position: absolute;
+    top: 8px;
+    right: 8px;
+    width: 28px;
+    height: 28px;
+    border: none;
+    border-radius: 50%;
+    background: rgba(0, 0, 0, 0.6);
+    color: white;
+    font-size: 18px;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    transition: background 0.2s;
+}
+
+.clear-image-btn:hover {
+    background: var(--danger);
+}
+
 /* Output */
 .output-video {
     width: 100%;
@@ -673,7 +701,7 @@ body {
     color: white;
 }
 
-.privacy-mode .preview-image,
+.privacy-mode .preview-container .preview-image,
 .privacy-mode .output-video,
 .privacy-mode .gallery-item-media video,
 .privacy-mode .gallery-item-media img {