Fix language switcher dropping query params (e.g. ?tool= on mcp-tool.php)

Preserve all existing GET params when building lang switcher URLs so that
switching language on /mcp-tool.php?tool=dbn.redact stays on that tool.

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

includes/nav.php

@@ -69,8 +69,13 @@ $_navAssetBase = str_contains($_navScriptPath, '/dashboard/') ? '../assets' : 'a
 
     <div class="dbn-nav__right">
         <nav class="dbn-nav__langs" aria-label="Language">
-<?php foreach (dbnToolsSupportedLanguages() as $lc): ?>
-            <a href="<?= htmlspecialchars($_navPath . '?lang=' . $lc) ?>"
+<?php
+    $_navQueryBase = $_GET;
+    foreach (dbnToolsSupportedLanguages() as $lc):
+        $_navQueryBase['lang'] = $lc;
+        $_navLangHref = $_navPath . '?' . http_build_query($_navQueryBase);
+?>
+            <a href="<?= htmlspecialchars($_navLangHref) ?>"
                class="dbn-nav__lang<?= $lc === $_navLang ? ' is-active' : '' ?>"
                hreflang="<?= htmlspecialchars($lc) ?>"
                aria-label="<?= htmlspecialchars(dbnToolsLanguageName($lc)) ?>"