Hide empty diagram containers and fix offline entry borders

- Hide .dict-word-family and .dict-semantic-map when empty (Mermaid
  failed to render) to avoid a stray border-top line at the bottom
- Switch offline entries from border-bottom to border-top on siblings
  to avoid a trailing line after the last entry

Author: liuchengxu

crates/markdown_preview_core/js/dictionary.css

@@ -411,7 +411,7 @@
     .dict-source-badge.offline { background: #0d2240; color: #58a6ff; }
     .dict-source-badge.online { background: #0d2d1a; color: #3fb950; }
     .dict-source-badge.ai { background: #3d2e00; color: #d29922; }
-    .dict-offline-entry { border-bottom-color: #30363d; }
+    .dict-offline-entry + .dict-offline-entry { border-top-color: #30363d; }
     .dict-entry { border-top-color: #30363d; }
     .dict-ai-loading { color: #8b949e; }
 
@@ -462,9 +462,13 @@
 
 /* Offline dictionary entry container */
 .dict-offline-entry {
-    margin-bottom: 20px;
-    padding-bottom: 20px;
-    border-bottom: 1px solid #d0d7de;
+    margin-bottom: 0;
+    padding-bottom: 0;
+}
+.dict-offline-entry + .dict-offline-entry {
+    margin-top: 20px;
+    padding-top: 20px;
+    border-top: 1px solid #d0d7de;
 }
 .dict-offline-content { font-size: 14px; line-height: 1.6; overflow: auto; position: relative; }
 
@@ -536,6 +540,10 @@
     padding-top: 16px;
     border-top: 1px solid #d0d7de;
 }
+.dict-word-family:empty,
+.dict-semantic-map:empty {
+    display: none;
+}
 .dict-diagram-label {
     font-size: 12px;
     font-weight: 700;