fix(ui): a dialog opens focused where it said it would, and the wizard is a dialog

Deep-review round 5, over round 4's own fixes.

- Round 4 gave `ui/Modal` a persistent portal host so a fullscreen transition would stop tearing
  the dialog down. The host was attached in a `useLayoutEffect` — one commit phase AFTER React
  runs `autoFocus` in `commitMount`, and `focus()` on a disconnected node is a no-op. Every
  autoFocus in every dialog was dead: Mod+K opened the command palette with the focus on the ✕,
  so typing searched nothing and Enter closed it; the same for the download rename field and
  every non-danger confirm. The host is attached in a `useInsertionEffect` now, a whole phase
  earlier, and the whole re-parent — including the focus save/restore across the move, which
  `appendChild` needs because it is remove-then-insert — lives in one `useFullscreenPortalHost`.
  `ui/AnchoredPopover` had a second copy of that re-parent WITHOUT the focus restore, and a
  comment claiming focus survived it; both now come from the hook.

- `ui/Toaster` classified a toast raised in the SAME commit as a host takeover among the ones it
  inherits, because it read the store live at the takeover render. A dialog that notifies and
  closes in one handler (UploadDialog) therefore put its toast up with no role, into a region
  still `aria-live="off"` — announced by neither path. "Inherited" is now what the outgoing host
  last had on screen, and a toast that is NOT inherited is announced even inside the unarmed
  window.

- The onboarding wizard renders its own shell, and Escape ordering was all of the dialog contract
  it had: no role, no label, no initial focus, no trap, no restore. A keyboard user tabbed out of
  the first-login wizard into the app behind the scrim. The focus half of the contract is now one
  shared `ui/dialogFocus` that both `ui/Modal` and the wizard use.

Every fix has a regression test, each mutation-checked (6 mutations, all red on the assertion
they are named after).

frontend/src/components/ui/AnchoredPopover.tsx

@@ -3,7 +3,6 @@ import {
   useContext,
   useEffect,
   useId,
-  useLayoutEffect,
   useRef,
   useState,
   type ButtonHTMLAttributes,
@@ -37,7 +36,7 @@ import {
   type UseRoleProps,
 } from "@floating-ui/react";
 import { useLayer } from "../../lib/layerStack";
-import { useFullscreenElement } from "../../lib/fullscreen";
+import { useFullscreenPortalHost } from "../../lib/fullscreen";
 
 // R8 popover platform. ONE anchored-popover primitive over @floating-ui/react, replacing the ~9
 // hand-rolled positioning engines (measured flip / width clamp / rAF scroll-follow / scroller-relative
@@ -528,19 +527,14 @@ function PopoverSurfaceInner({
 
   // A node portaled to <body> is painted UNDERNEATH whatever holds the fullscreen lock, so a popover
   // opened from a player's control bar in fullscreen would be invisible; it must portal INTO the
-  // fullscreen element. We own one persistent container div and re-parent it (body ↔ fullscreen
-  // element) with appendChild, which MOVES the node atomically — no detach, so the open panel's
-  // focus, caret, scroll offset and animation state survive a fullscreen transition, and an element
-  // SWAP (A→B without exiting) is handled by the element-identity dependency. (Keying the portal to
-  // remount instead would rebuild the subtree and lose all of that.)
-  const fsRoot = useFullscreenElement();
-  const [container] = useState<HTMLDivElement | null>(() =>
-    typeof document === "undefined" ? null : document.createElement("div"),
-  );
-  useLayoutEffect(() => {
-    if (container) (fsRoot ?? document.body).appendChild(container);
-  }, [container, fsRoot]);
-  useEffect(() => () => container?.remove(), [container]);
+  // fullscreen element. One persistent container div is re-parented (body ↔ fullscreen element)
+  // rather than handed to the portal as a changing container, so the open panel's caret, scroll
+  // offset and animation state survive the transition, and an element SWAP (A→B without exiting) is
+  // handled by the element-identity dependency. (Keying the portal to remount instead would rebuild
+  // the subtree and lose all of that.) Focus does NOT survive on its own — `appendChild` on an
+  // already-parented node is remove-then-insert, which blurs to <body>; the hook saves and restores
+  // it. This used to be a second copy of Modal's re-parent that lacked exactly that (round 5).
+  const container = useFullscreenPortalHost();
 
   const { style: floatingPropStyle, ...floatingProps } = getFloatingProps() as {
     style?: CSSProperties;