feat(admin): pre-select the only source blog when exactly one exists

If the current blog has just one MSLS-active sibling, the 'pick a
source' step adds no value — the list is already unambiguous. Treats
that single source as selected when the URL has no msls_source set, so
the user lands directly on the list of untranslated posts.

The source flag for that blog still renders in the active state, so the
UI truthfully reflects which blog is driving the list.

Author: apermo

includes/MslsTranslationPickerPage.php

@@ -224,6 +224,12 @@ public static function render(): void {
 			}
 		}
 
+		// With only one source available, treat it as pre-selected so the
+		// user lands straight on the list instead of a picker-of-one.
+		if ( 0 === $source && 1 === count( $blogs ) ) {
+			$source = (int) $blogs[0]->userblog_id;
+		}
+
 		self::enqueue( (int) get_current_blog_id() );
 
 		echo '<div class="wrap msls-tp-page">';