Show staged pins in the tree

A staged pin was only visible in the detail pane, so the header's
"n pin staged" could not be traced to a row. The status column now shows
a blue "staged" instead of "in sync", a blue marker after an issue word,
and the marker on collapsed rows with a staged pin below. It is still not an
issue and not counted in +n.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/state-model.md

@@ -82,6 +82,8 @@ Rules:
   changed.
 - **Not issues** (quiet): in sync, pin behind/ahead/diverged but pushed,
   staged pin, detached HEAD on the pin. They are shown as information only.
+  A staged pin is also visible in the tree (`staged` or a `●` marker), see
+  [ux-guide.md](ux-guide.md#tree-row), but is never counted as an issue.
 - A loading error is shown as `error` (Danger) in the UI; it is not an issue
   kind.