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/playerModalControls.tsx

@@ -1,6 +1,7 @@
 import { useTranslation } from "react-i18next";
 import { Repeat, Repeat1, Shuffle, SkipBack, SkipForward } from "lucide-react";
 import type { AutoMode, LoopMode } from "../lib/playerModes";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Presentational controls for the YouTube modal player (PlayerModal), split out of the god component —
 // the sibling of plexPlayerControls.tsx. No state or effects here; all behaviour comes in via props.
@@ -51,7 +52,7 @@ export function PlayerTransportControls({
   const toggleLabel = docked ? "hidden md:inline" : "";
   return (
     <>
-      <button
+      <FocusSafeButton
         onClick={onPrev}
         disabled={index === 0}
         title={`${t("player.previous")} · Shift+←`}
@@ -59,11 +60,11 @@ export function PlayerTransportControls({
       >
         <SkipBack className="h-4 w-4" />
         <span className={navLabel}>{t("player.previous")}</span>
-      </button>
+      </FocusSafeButton>
       <span className={counter}>
         {index + 1} / {queueLength}
       </span>
-      <button
+      <FocusSafeButton
         onClick={onNext}
         disabled={index === queueLength - 1}
         title={`${t("player.next")} · Shift+→`}
@@ -71,7 +72,7 @@ export function PlayerTransportControls({
       >
         <span className={navLabel}>{t("player.next")}</span>
         <SkipForward className="h-4 w-4" />
-      </button>
+      </FocusSafeButton>
       <span className={divider} />
       {/* Persistent playback settings — cycle on click, saved to your account. */}
       <button