feat(dm): clickable route popup for long delivery paths

Long routes (>4 hops) show truncated with dotted underline; clicking
opens a popup with the full route and hop count, same style as channel
message path popups. Short routes (<=4 hops) display inline as before.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/static/css/style.css

@@ -750,6 +750,13 @@ main {
     margin-top: 0.1rem;
 }
 
+.dm-route-link {
+    cursor: pointer;
+    text-decoration: underline;
+    text-decoration-style: dotted;
+    position: relative;
+}
+
 .dm-delivery-popup {
     position: absolute;
     bottom: 100%;