refactor(ui): the popover engine and every render-phase ref write compile under the React Compiler

E32 S3. The compiler skipped 93 functions; it now skips 29 (try/finally shapes, two suppressions,
VirtualGrid). react-hooks/refs hits 245 -> 31.

- Popover engine: `ref={pop.refs.setReference}` made the compiler treat the whole popover handle as
  a ref, so every component holding one was skipped (21 call sites, the engine and its surface
  among them). The setter is destructured now; the same for useScrollFade and useReturnFocus.
- Render-phase `ref.current = x` writes moved to `useEffectEvent` where only effects read them,
  else to the new `useLatest` (written in a layout effect) / `useStableCallback`.
- FocusSafeButton (under every Button) read the focused element during render; it reads it through
  useSyncExternalStore, which React asks on every render instead of a cached memo.

Bugs this surfaced, fixed:
- Combobox: a chip could keep showing its raw id. The label cache was a ref an effect filled after
  the render that needed it; it is state merged during render now (test seen RED).
- useUndoable rendered from a ref plus a forced re-render; compiled, that memoizes a stale value.
  The history is state now, mirrored in a ref for the stable callbacks.
- ConfigPanel and the saved-views editor re-seeded their drafts in an effect, painting the stale
  draft for a frame; both adjust during render against the remembered key.
- Toaster's handover is written after commit instead of during render; a new test pins that the
  inherited set is taken once, at the takeover (seen RED on a per-render re-read).
- IssueComposer mutated a ref prop as a one-shot focus token; it is plain state.
- PlexBrowse wrote its info-scroll memory from an effect declared before the ref, past its own
  "one writer" rule; the writer comes first now and the effect goes through it.

The compiler ratchet drops to the measured 29 and now also requires FocusSafeButton and the popover
engine to compile.

frontend/src/components/ui/sortableA11y.ts

@@ -1,4 +1,5 @@
-import { useMemo, useRef } from "react";
+import { useMemo } from "react";
+import { useLatest } from "../../lib/useLatest";
 import { useTranslation } from "react-i18next";
 import { closestCenter } from "@dnd-kit/core";
 import type {
@@ -38,8 +39,7 @@ export function useSortableA11y(describe: (id: UniqueIdentifier) => SortableSubj
   announcements: Announcements;
 } {
   const { t } = useTranslation();
-  const latest = useRef(describe);
-  latest.current = describe;
+  const latest = useLatest(describe);
   return useMemo(() => {
     // Spread into a fresh object: i18next's options type is an index signature, which a named
     // interface does not satisfy.
@@ -51,22 +51,21 @@ export function useSortableA11y(describe: (id: UniqueIdentifier) => SortableSubj
     // dnd-kit reports the item over ITSELF right after the pick-up, in the same tick — announced,
     // "moved to position 1" replaced "picked up … use the arrow keys" in the live region before a
     // screen reader read it (measured in Chrome). Only a real move is announced as one.
-    let moved = false;
+    // Per-DRAG state the announcement closures share (a component-lifetime ref would be the wrong
+    // lifetime): an object owned by this memo, so the closures mutate a property, never a binding.
+    const drag = { moved: false };
     return {
       screenReaderInstructions: { draggable: t("common.dnd.instructions") },
       announcements: {
         onDragStart: ({ active }) => {
-          moved = false;
+          drag.moved = false;
           return t("common.dnd.pickedUp", at(active.id));
         },
         onDragOver: ({ active, over }) => {
-          if (!over || (!moved && over.id === active.id)) return undefined;
-          // `moved` is per-DRAG state, and the announcement closures capturing it is the point:
+          if (!over || (!drag.moved && over.id === active.id)) return undefined;
           // dnd-kit reports the item over ITSELF in the pick-up tick (see above), and this flag is
-          // the only thing separating that from a real move. A ref would be the wrong lifetime —
-          // one drag, not the component.
-          // eslint-disable-next-line react-hooks/immutability -- per-DRAG state the announcement closures share; a ref would live as long as the component
-          moved = true;
+          // the only thing separating that from a real move.
+          drag.moved = true;
           return t("common.dnd.movedOver", landing(active.id, over.id));
         },
         onDragEnd: ({ active, over }) =>
@@ -76,7 +75,7 @@ export function useSortableA11y(describe: (id: UniqueIdentifier) => SortableSubj
         onDragCancel: ({ active }) => t("common.dnd.cancelled", at(active.id)),
       },
     };
-  }, [t]);
+  }, [t, latest]);
 }
 
 // A context that nests sortable lists (a Plex playlist: its blocks, and each show's episodes) must