Show a muted "in sync" for submodules without issues

An empty status cell could be read as missing data. The muted word
confirms the check ran without drawing attention. The word is defined
once in the theme and shared with the detail pane.

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

internal/tui/theme/issues.go

@@ -9,6 +9,9 @@ import (
 	"submarine/internal/submodule"
 )
 
+// InSync is the word for a submodule without issues.
+const InSync = "in sync"
+
 // IssueWord is the short label for an issue kind shown in the tree.
 func IssueWord(k submodule.IssueKind) string {
 	switch k {