fix(tui): move theme styles onto the Model to end cross-session races

applyTheme mutated ~18 package-global lipgloss styles while every SSH
session's tea.Program read them concurrently from its own goroutine.
Pressing T or opening a new connection raced other sessions' View and
bled themes across users.

Styles now live in an immutable per-Model struct built by newStyles;
free formatter helpers that consumed the globals became Model methods.

internal/tui/update_test.go

@@ -99,6 +99,8 @@ func newTestModel(ms *tuiMockStore) Model {
 		isAdmin:             true,
 		zones:               zone.New(),
 		detailChangesSiteID: -1,
+		theme:               themeFlexokiDark,
+		st:                  newStyles(themeFlexokiDark),
 	}
 }