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/plexBrowseLists.tsx

@@ -12,6 +12,7 @@ import { LoadingState, StateMessage } from "./ui/QueryState";
 import PosterTile from "./PosterTile";
 import PlexLibraryPicker from "./PlexLibraryPicker";
 import NavGrid from "./ui/NavGrid";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // A collection tile's sub-line: item count · total runtime (each dropped when unknown, e.g. show-only
 // collections have no movie runtime).
@@ -145,14 +146,14 @@ function NewCollectionPanel({
         term={term}
         enabled={nameOk}
         renderAction={(c) => (
-          <button
+          <FocusSafeButton
             onClick={() => create(c)}
             disabled={busy}
             className="glass-card glass-hover inline-flex items-center gap-1 rounded-lg px-2.5 py-1 text-xs disabled:opacity-50"
           >
             <Check className="h-3.5 w-3.5" />
             {t("plex.collection.createWith")}
-          </button>
+          </FocusSafeButton>
         )}
       />
     </div>