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/ChatDock.tsx

@@ -7,6 +7,7 @@ import { onMessage, onUnread } from "../lib/messagesSocket";
 import * as e2ee from "../lib/e2ee";
 import Avatar from "./Avatar";
 import ChatThread from "./ChatThread";
+import Overlay from "./Overlay";
 
 // The Messenger-style floating chat dock: open conversations live here, bottom-right, across
 // page navigation (state persisted per user). Always mounted (for human users) so it also owns
@@ -51,11 +52,13 @@ export default function ChatDock({ meId }: { meId: number }) {
 
   if (chats.length === 0) return null;
   return (
-    <div className="fixed bottom-0 right-0 z-40 flex flex-row-reverse items-end gap-3 p-4 pointer-events-none">
-      {chats.map((c) => (
-        <ChatWindow key={c.partnerId} chat={c} meId={meId} />
-      ))}
-    </div>
+    <Overlay>
+      <div className="fixed bottom-0 right-0 z-rail flex flex-row-reverse items-end gap-3 p-4 pointer-events-none">
+        {chats.map((c) => (
+          <ChatWindow key={c.partnerId} chat={c} meId={meId} />
+        ))}
+      </div>
+    </Overlay>
   );
 }
 
@@ -65,7 +68,7 @@ function ChatWindow({ chat, meId }: { chat: DockChat; meId: number }) {
     <div className="pointer-events-auto relative w-80 max-w-[calc(100vw-2rem)] glass rounded-2xl shadow-2xl flex flex-col overflow-hidden">
       {/* One-shot attention flash on a new message; keyed by the counter so it replays each time. */}
       {chat.flash > 0 && (
-        <div key={chat.flash} className="chat-flash pointer-events-none absolute inset-0 rounded-2xl z-10" />
+        <div key={chat.flash} className="chat-flash pointer-events-none absolute inset-0 rounded-2xl z-base" />
       )}
       <div className="flex items-center gap-2 px-3 py-2 border-b border-border bg-card/50">
         <button