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/plexViews.tsx

@@ -16,7 +16,7 @@ import {
   DetailHero,
   Filterable,
   WatchedToggle,
-} from "../lib/plexDetailUi";
+} from "./plexDetailUi";
 import { epLabel, tmdbBackdropUrl, useCrossfadeBackdrop, useDetailPrefs } from "../lib/plexDetail";
 import { CastStrip, EpisodeCard, PlexPosterCard, RelatedStrip, SeasonCard } from "./plexCards";
 import { PlexPinMenu } from "./ComfortPinMenu";
@@ -31,7 +31,7 @@ import NavGrid from "./ui/NavGrid";
 // The Plex sub-router's view branches, split out of PlexBrowse. Each is invoked by PlexBrowse with
 // shell callbacks as props (onBack/onPlay/onFilter/…) — no view reaches back into the shell's state,
 // which is what made this a pure code-motion split. Cards/strips come from plexCards, the shared
-// detail-page UI from lib/plexDetailUi.
+// detail-page UI from plexDetailUi.
 
 // Lazy: the rich movie info panel loads only when an info page is opened.
 const PlexInfo = lazy(() => import("./PlexInfo"));