Workaround Qt's lack of support for HTML <hr>

When processing message HTML for the client from python, turn <hr>
tags into <p>──────...</p> em dash lines

src/backend/html_markdown.py

@@ -359,6 +359,7 @@ class HTMLProcessor:
             ],
             "element_postprocessors": [
                 self._font_color_to_span if outgoing else lambda el: el,
+                self._hr_to_dashes if not outgoing else lambda el: el,
             ],
             "is_mergeable": lambda e1, e2: e1.attrib == e2.attrib,
         }
@@ -555,4 +556,14 @@ class HTMLProcessor:
         return el
 
 
+    def _hr_to_dashes(self, el: HtmlElement) -> HtmlElement:
+        if el.tag != "hr":
+            return el
+
+        el.tag             = "p"
+        el.attrib["class"] = "ruler"
+        el.text            = "─" * 19
+        return el
+
+
 HTML_PROCESSOR = HTMLProcessor()