fix(ui): Size Select popups to the widest option and truncate the PR repo name

Virtualised list rows are absolutely positioned, so the popup only took the
trigger's width and cut short labels like "Merged". The Pull Requests header
now keeps the repository on one line with an ellipsis and a tooltip.

src/features/github/Github.module.css

@@ -37,7 +37,11 @@
   white-space: nowrap;
 }
 .repo {
+  min-width: 0;
   margin-left: 8px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
   color: var(--fg-subtle);
   font-size: var(--fs-small);
 }