fix: popout theme inheritance via URL query param

- Pass current theme as ?theme=dark|light in popout URL
- viewer.js, app.js, palette.js all pass theme to popout
- popout.html reads theme from URL param first (fallback: localStorage, then dark)
- Also syncs theme back to localStorage when set via query param

frontend/popout.html

@@ -5,7 +5,13 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>ObsiGate - Popout</title>
   <script>
-    document.documentElement.setAttribute('data-theme', localStorage.getItem('obsigate-theme') || 'dark');
+    // Inherit theme from opener or localStorage — query param has priority
+    (function() {
+      var m = location.search.match(/theme=([^&]+)/);
+      var theme = (m && m[1]) || localStorage.getItem('obsigate-theme') || 'dark';
+      document.documentElement.setAttribute('data-theme', theme);
+      if (m && m[1]) { localStorage.setItem('obsigate-theme', theme); }
+    })();
   </script>
   <link rel="stylesheet" href="/static/style.css">
   <script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>