Fix <sql> silent-failure bug; split /vst into overview + subpage; misc site fixes

- __mysql.cpp: <sql> never checked mysql_real_query's own return code,
  so a genuine query failure was indistinguishable from "ran fine, zero
  rows" - <try>/<catch> around a failing <sql> never actually caught
  anything anywhere. Now throws on a real failure.
- spa.js: dynamically re-injected scripts now run in true sequential
  order (wait for each external script's load event before running the
  next), fixing checkout's Square card field never building on the
  first client-side navigation to /checkout (only after a refresh) -
  async=false alone only orders multiple external scripts relative to
  each other, it doesn't delay a plain inline sibling.
- /vst split into a real overview + /vst/modular1 subpage with a left
  menu, reusing opensource.page's existing subpage-tier pattern
  (opensource.menu/main.css's submenuitem-active highlighting, and
  several other subpage-tier retirements, were already finished earlier
  this session but never committed - included here too).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

__mysql.cpp

@@ -157,6 +157,18 @@ namespace jet {
 
    void __mysql::query(coreutils::MString query) {
       int rc = mysql_real_query(mysql, query.getData(), query.getLength());
+      // rc was captured but never checked - a genuinely failed query
+      // (syntax error, constraint violation, etc.) fell straight into
+      // the result==NULL branch below, which is ALSO the normal path
+      // for a successful non-SELECT statement (INSERT/UPDATE/DELETE
+      // have no result set to store either). That made every SQL error
+      // in the whole codebase silently indistinguishable from "ran fine,
+      // no rows" - <try>/<catch> around a failing <sql> never caught
+      // anything, since no exception was ever thrown in the first place.
+      // Only rc (mysql_real_query's own success/failure signal) can
+      // actually tell the two cases apart; result's nullness can't.
+      if(rc != 0)
+	throw coreutils::Exception(std::string("mysql query failed: ") + mysql_error(mysql));
       result = mysql_store_result(mysql);
       if(result) {
 	 row = mysql_fetch_row(result);