fix(ui): no button drops the keyboard focus when its own action disables it

A focused button that turns natively disabled loses the focus to <body> in
Chrome. The app disables the very button that was pressed on ~60 sites — a
mutation pending, a pager at its last page, undo with nothing left, a player
at the end of its queue, a form gone clean — so a keyboard user was thrown
out of the page on every such press.

One primitive instead of a fix per site: ui/FocusSafeButton renders a
focused-and-disabled (or busy) button as aria-disabled with a click guard,
and applies the native attribute again once the focus is elsewhere. It asks
the document at render time as well as listening for focus events, because
a window without focus fires none (measured). ui/Button and ui/form's Switch
are built on it; 57 hand-rolled buttons swapped their tag for it keeping
their classes, and three hand-copies of the rule (the channel page's Block
and Hide, ChannelDiscovery's Subscribe) now pass `busy`. The soft state's
dim look is one rule in index.css.

`local/focus-safe-disabled` refuses a new raw <button disabled>, pinned by
focusSafe.contract.test.ts. lib/plexDetailUi moved to components/: it is
components, and lib may not import the primitive.

Measured on localdev in Chrome: the audit log's Next button keeps the focus
on reaching the last page, reads aria-disabled, and the guarded click does
nothing.

frontend/src/components/Feed.tsx

@@ -50,6 +50,7 @@ import FilterSelect from "./ui/FilterSelect";
 import ItemCount from "./ui/ItemCount";
 import ViewSwitcher from "./ViewSwitcher";
 import VirtualFeed from "./VirtualFeed";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 // Lazy: the in-app YouTube player (IFrame API) loads only when a video is first opened.
 const PlayerModal = lazy(() => import("./PlayerModal"));
 
@@ -516,7 +517,7 @@ export default function Feed({
             />
             {ytQuery.hasNextPage && (
               <div className="text-center pt-4">
-                <button
+                <FocusSafeButton
                   onClick={() => ytQuery.fetchNextPage()}
                   disabled={ytQuery.isFetchingNextPage}
                   className="inline-flex items-center gap-2 px-4 py-2 rounded-xl border border-border bg-card text-sm font-medium hover:border-accent hover:text-accent disabled:opacity-50 transition"
@@ -526,7 +527,7 @@ export default function Feed({
                     : zeroQuota
                       ? t("feed.yt.loadMoreFree")
                       : t("feed.yt.loadMore")}
-                </button>
+                </FocusSafeButton>
               </div>
             )}
           </>