frontend: print nothing under an empty paragraph

An empty paragraph printed 「（本段暂无内容）」 under its heading. A paragraph
with no sentences is not an error to report: it is a position in an outline, and
some of those are meant to be empty — a separator, a position the template
reserves, a section the writer has not reached yet. The words also go with the
paper on export, which is worse: a placeholder is the tool talking, inside the
document.

The structure stays exactly as it was (the heading, the count, the edit button);
only the annotation is gone. Measured on a paper with 19 empty paragraphs: no
placeholder element anywhere, the text absent from the page, and each empty
paragraph renders as its heading alone at 56px.

frontend/src/components/papers/PaperParagraph.vue

@@ -5,8 +5,10 @@
  * Three things about this component carry the feature's contracts:
  *
  * 1. **The structure is never skipped.** A paragraph with no sentences still
- *    renders its heading and a quiet placeholder, because the paper's shape
- *    comes from its template and an unwritten paragraph is still a paragraph.
+ *    renders its heading, because the paper's shape comes from its template and
+ *    an unwritten paragraph is still a paragraph — and then prints nothing
+ *    under it. A blank paragraph is blank: some positions in an outline are
+ *    meant to stay empty, and placeholder text would end up in the paper.
  * 2. **A paragraph is reassembled from its sentences.** They are printed in
  *    `sort` order, and the only thing between them is the server's
  *    `separator_before` — a space for English, nothing for Chinese. This
@@ -117,7 +119,13 @@ const anchor = computed(() => `paragraph-${props.paragraph.paper_template_filed_
       </template>
     </p>
 
-    <p v-else class="paragraph-empty">（本段暂无内容）</p>
+    <!-- Nothing below the heading when there is nothing to print. An empty
+         paragraph is a position in an outline, and in an academic paper some
+         of those are meant to be empty — a separator, a placeholder the
+         template reserves, a section the writer has not reached yet. Printing
+         「（本段暂无内容）」 into a paper puts words there that the paper did
+         not ask for, and on export they would go with it. The blank body is
+         the honest rendering of a blank paragraph. -->
   </section>
 </template>
 
@@ -213,13 +221,6 @@ const anchor = computed(() => `paragraph-${props.paragraph.paper_template_filed_
   line-height: 0;
 }
 
-.paragraph-empty {
-  margin: 8px 0 0;
-  font-size: 13px;
-  color: var(--el-text-color-placeholder);
-  font-style: italic;
-}
-
 .cite-tip {
   max-width: 320px;
 }

frontend/src/views/papers/PaperDetailView.vue

@@ -11,8 +11,9 @@
  * Three states are visible and none of them is an error:
  *
  * * a **written paragraph** — heading, prose, numbered citation markers;
- * * an **empty paragraph** — heading and a placeholder, because the structure
- *   is there before the words are;
+ * * an **empty paragraph** — its heading and nothing else. Some positions in an
+ *   outline are meant to stay empty, so a blank paragraph is printed blank
+ *   rather than annotated;
  * * an **unmatched paragraph** (未设定) — content whose position the current
  *   template does not define, kept in order rather than hidden, and recoverable
  *   by editing the paragraph or switching back.