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/Stats.tsx

@@ -16,6 +16,7 @@ import Tabs from "./ui/Tabs";
 import { Section, SettingRow } from "./ui/form";
 import { useMe } from "../lib/useMe";
 import { notifyActionError } from "../lib/actionErrors";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 const RANGES = [7, 30, 90] as const;
 type StatsTab = "overview" | "system";
@@ -175,24 +176,24 @@ function Overview({ me }: { me: Me }) {
       {!me.is_demo && (
         <Section title={t("stats.sync.actions")}>
           <Tooltip hint={t("stats.sync.syncSubscriptionsHint")}>
-            <button
+            <FocusSafeButton
               onClick={() => syncSubs.mutate()}
               disabled={syncSubs.isPending}
               className="glass-card glass-hover flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
             >
               <RefreshCw className={`w-4 h-4 ${syncSubs.isPending ? "animate-spin" : ""}`} />
               {t("stats.sync.syncSubscriptions")}
-            </button>
+            </FocusSafeButton>
           </Tooltip>
           <Tooltip hint={t("stats.sync.backfillEverythingHint")}>
-            <button
+            <FocusSafeButton
               onClick={() => deepAll.mutate()}
               disabled={deepAll.isPending}
               className="glass-card glass-hover flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition mt-2"
             >
               <History className={`w-4 h-4 ${deepAll.isPending ? "animate-pulse" : ""}`} />
               {t("stats.sync.backfillEverything")}
-            </button>
+            </FocusSafeButton>
           </Tooltip>
         </Section>
       )}