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/Scheduler.tsx

@@ -26,6 +26,7 @@ import { formatNumber, relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./ui/Tooltip";
 import { notifyActionError } from "../lib/actionErrors";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 const POLL_MS = 4000;
 // While any job is running, poll faster so short scheduled runs aren't missed between ticks.
@@ -177,14 +178,14 @@ function JobRow({
                 className="w-16 bg-surface border border-border rounded-sm px-1.5 py-0.5 text-xs focus:outline-hidden focus:ring-2 focus:ring-accent/40"
               />
               <span className="text-muted text-xs">{t("scheduler.minutes")}</span>
-              <button
+              <FocusSafeButton
                 onClick={save}
                 disabled={saving}
                 className="text-success hover:opacity-80 disabled:opacity-50"
                 aria-label={t("common.save")}
               >
                 <Check className="w-3.5 h-3.5" />
-              </button>
+              </FocusSafeButton>
               <button
                 onClick={() => setEditing(false)}
                 className="text-muted hover:text-fg"
@@ -232,14 +233,14 @@ function JobRow({
         )}
       </div>
       <Tooltip hint={t("scheduler.runNow")}>
-        <button
+        <FocusSafeButton
           onClick={onRun}
           disabled={job.running || runDisabled}
           aria-label={t("scheduler.runNow")}
           className="shrink-0 p-1.5 rounded-lg text-muted hover:text-accent hover:bg-card transition disabled:opacity-30 disabled:hover:text-muted disabled:hover:bg-transparent"
         >
           <Play className="w-4 h-4" />
-        </button>
+        </FocusSafeButton>
       </Tooltip>
     </div>
   );
@@ -464,17 +465,17 @@ export default function Scheduler() {
         </div>
         <div className="flex-1" />
         <Tooltip hint={data.paused ? t("scheduler.runAllPausedHint") : t("scheduler.runAllHint")}>
-          <button
+          <FocusSafeButton
             onClick={() => runAllMut.mutate()}
             disabled={runAllMut.isPending || data.paused}
             className="glass-card glass-hover flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
           >
             <Play className="w-4 h-4" />
             {t("scheduler.runAll")}
-          </button>
+          </FocusSafeButton>
         </Tooltip>
         <Tooltip hint={t("scheduler.purgeDiscoveryHint")}>
-          <button
+          <FocusSafeButton
             onClick={async () => {
               const ok = await confirm({
                 title: t("scheduler.purgeConfirmTitle"),
@@ -489,17 +490,17 @@ export default function Scheduler() {
           >
             <Trash2 className="w-4 h-4" />
             {t("scheduler.purgeDiscovery")}
-          </button>
+          </FocusSafeButton>
         </Tooltip>
         <Tooltip hint={t("scheduler.pauseHint")}>
-          <button
+          <FocusSafeButton
             onClick={() => pauseResume.mutate(data.paused)}
             disabled={pauseResume.isPending}
             className="glass-card glass-hover flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
           >
             {data.paused ? <Play className="w-4 h-4" /> : <Pause className="w-4 h-4" />}
             {data.paused ? t("scheduler.resume") : t("scheduler.pause")}
-          </button>
+          </FocusSafeButton>
         </Tooltip>
       </div>