Fix popstate and client routing when no [sx-boost] container exists

handle-popstate falls back to #main-panel when no [sx-boost] element
is found, fixing back button for apps using explicit sx-target attrs.
bindClientRouteClick also checks sx-target on the link itself.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/bootstrap_js.py

@@ -2616,9 +2616,12 @@ PLATFORM_ORCHESTRATION_JS = """
     link.addEventListener("click", function(e) {
       e.preventDefault();
       var pathname = urlPathname(href);
-      // Find the sx-boost target selector from the link's ancestor
+      // Find target selector: sx-boost ancestor, explicit sx-target, or #main-panel
       var boostEl = link.closest("[sx-boost]");
       var targetSel = boostEl ? boostEl.getAttribute("sx-boost") : null;
+      if (!targetSel || targetSel === "true") {
+        targetSel = link.getAttribute("sx-target") || "#main-panel";
+      }
       if (tryClientRoute(pathname, targetSel)) {
         try { history.pushState({ sxUrl: href, scrollY: window.scrollY }, "", href); } catch (err) {}
         if (typeof window !== "undefined") window.scrollTo(0, 0);