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>

internal/tui/theme/issues.go

@@ -12,6 +12,10 @@ import (
 // InSync is the word for a submodule without issues.
 const InSync = "in sync"
 
+// Staged is the word for a pin change staged in the superproject. It is not
+// an issue: it replaces InSync, or follows an issue word as a marker.
+const Staged = "staged"
+
 // IssueWord is the short label for an issue kind shown in the tree.
 func IssueWord(k submodule.IssueKind) string {
 	switch k {