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/Playlists.tsx

@@ -68,6 +68,7 @@ import { ColumnHead } from "./ui/tableHeader";
 import { alignClass } from "./ui/tableAlign";
 import { filterRows, isActive, type Column, type FilterMap } from "../lib/tableColumns";
 import { GRIP_KEY, playlistColumns } from "./playlistColumns";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // The playlist detail is a HYBRID table (E4 S4): real sortable/filterable column headers over a
 // body that is still a drag-reorderable @dnd-kit list.
@@ -936,15 +937,15 @@ export default function Playlists() {
                         {t("playlists.unsynced")}
                       </span>
                     )}
-                    <button
+                    <FocusSafeButton
                       onClick={() => shown.length && openPlayer(shown[0]!.id)}
                       disabled={!shown.length}
                       className="inline-flex items-center gap-1.5 text-xs font-semibold px-3 py-1.5 rounded-lg btn-primary disabled:opacity-40 transition"
                     >
                       <Play className="w-3.5 h-3.5 fill-current" /> {t("playlists.playAll")}
-                    </button>
+                    </FocusSafeButton>
                     {editable && canWrite && (
-                      <button
+                      <FocusSafeButton
                         onClick={pushToYoutube}
                         disabled={pushing}
                         title={
@@ -966,7 +967,7 @@ export default function Playlists() {
                         ) : (
                           <Upload className="w-3.5 h-3.5" />
                         )}
-                      </button>
+                      </FocusSafeButton>
                     )}
                     {linked && detail.dirty && (
                       <Button