feat(comments): 评论区有内容时自动调淡背景图透明度

与 tiptap 编辑器保持一致：无内容时 60%，有内容或聚焦时淡出到
10%。content_md 由 oninput 实时维护，输入即触发重渲染。

src/components/comments/form.rs

@@ -155,10 +155,21 @@ pub fn CommentForm(post_id: i32, parent_id: Option<i64>, parent_indent: Option<i
                             disabled: submitting(),
                             oninput: move |e| content_md.set(e.value()),
                         }
-                        img {
-                            src: "/images/xiantiaoxiaogou_input_bg.webp",
-                            alt: "",
-                            class: "absolute bottom-1.5 right-1.5 w-24 pointer-events-none opacity-60 peer-focus:opacity-10 transition-opacity duration-300 z-0",
+                        {
+                            // 无内容时半透明(60%)；有内容或聚焦时淡出(10%)。
+                            // content_md 由 oninput 实时维护，输入即触发重渲染。
+                            let bg_opacity = if content_md().trim().is_empty() {
+                                "opacity-60"
+                            } else {
+                                "opacity-10"
+                            };
+                            rsx! {
+                                img {
+                                    src: "/images/xiantiaoxiaogou_input_bg.webp",
+                                    alt: "",
+                                    class: "absolute bottom-1.5 right-1.5 w-24 pointer-events-none peer-focus:opacity-10 transition-opacity duration-300 z-0 {bg_opacity}",
+                                }
+                            }
                         }
                     }
                 }