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/PlexInfo.tsx

@@ -6,7 +6,7 @@ import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } f
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
 import { notifyActionError } from "../lib/actionErrors";
-import { DetailCustomizeMenu, Filterable, PrefToggle } from "../lib/plexDetailUi";
+import { DetailCustomizeMenu, Filterable, PrefToggle } from "./plexDetailUi";
 import { tmdbBackdropUrl, useCrossfadeBackdrop, useDetailPrefs } from "../lib/plexDetail";
 import { usePref } from "../lib/usePref";
 import i18n from "../i18n";
@@ -15,6 +15,7 @@ import PlexCollectionEditor from "./PlexCollectionEditor";
 import PlexPlaylistAdd from "./PlexPlaylistAdd";
 import { PLAYABLE_TINT, playableClass } from "./plexPlayable";
 import { navItemAttrs } from "../lib/listNav";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // The rich "media info" surface for a Plex item — poster/art, IMDb score + link, content rating,
 // genres, director, cast (with photos), summary. Reused in TWO places: a full-page view opened from
@@ -736,7 +737,7 @@ export default function PlexInfo({
               </div>
               <Carousel ariaLabel={col.title} gap="gap-3" keyboardNav={!!onPlayItem}>
                 {col.items.map((m, i) => (
-                  <button
+                  <FocusSafeButton
                     key={m.id}
                     {...navItemAttrs(i)}
                     onClick={() => onPlayItem?.(m.id)}
@@ -778,7 +779,7 @@ export default function PlexInfo({
                       {m.title}
                     </div>
                     {m.year ? <div className="text-2xs text-muted">{m.year}</div> : null}
-                  </button>
+                  </FocusSafeButton>
                 ))}
               </Carousel>
             </div>