refactor(overlay): single overlay-root convention via Overlay helper

Add components/Overlay.tsx (portals to <body>) and route the in-tree full-screen/
floating holdouts through it — ChatDock, OnboardingWizard, GlassTuner — so they
escape any ancestor stacking context (mask/transform/backdrop-filter) instead of
relying on a bare high z-index. Toaster stays in-tree (it's content-column-scoped
by design, outside the masked scroller); Welcome is a page, not an overlay.

frontend/src/components/GlassTuner.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useState } from "react";
+import Overlay from "./Overlay";
 
 /**
  * A floating live appearance tuner. Mounted at the app root so it's reachable on every page; it
@@ -216,7 +217,8 @@ export default function GlassTuner({ enabled }: { enabled: boolean }) {
   if (!enabled) return null;
 
   return (
-    <div className="fixed right-0 top-24 z-[9999] flex items-start" style={{ pointerEvents: "none" }}>
+    <Overlay>
+    <div className="fixed right-0 top-24 z-tuner flex items-start" style={{ pointerEvents: "none" }}>
         {!open && (
           <button
             onClick={() => persistOpen(true)}
@@ -335,6 +337,7 @@ export default function GlassTuner({ enabled }: { enabled: boolean }) {
           </div>
         )}
     </div>
+    </Overlay>
   );
 }