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/SetupWizard.tsx

@@ -6,6 +6,7 @@ import { setLanguage, type LangCode } from "../i18n";
 import LanguageSwitcher from "./LanguageSwitcher";
 import { Button } from "./ui/Button";
 import { inputCls } from "./ui/form";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // First-run install wizard. Shown by App.tsx while the instance is unconfigured. The one-time
 // setup token comes from the URL the app printed to the container logs (?token=…); every step
@@ -279,15 +280,15 @@ export default function SetupWizard() {
           )}
 
           <div className="flex items-center justify-between mt-6">
-            <button
+            <FocusSafeButton
               onClick={() => go(-1)}
               disabled={stepIdx === 0 || busy}
               className="glass-card glass-hover inline-flex items-center gap-1 px-3 py-2 rounded-xl text-sm disabled:opacity-40 transition"
             >
               <ChevronLeft className="w-4 h-4" />
               {t("setup.nav.back")}
-            </button>
-            <button
+            </FocusSafeButton>
+            <FocusSafeButton
               onClick={next}
               disabled={busy}
               className="inline-flex items-center gap-1.5 px-4 py-2 rounded-xl text-sm btn-primary font-medium shadow-md shadow-accent/20 disabled:opacity-50 transition"
@@ -299,7 +300,7 @@ export default function SetupWizard() {
               ) : null}
               {step === "finish" ? t("setup.nav.finish") : t("setup.nav.next")}
               {step !== "finish" && !busy && <ChevronRight className="w-4 h-4" />}
-            </button>
+            </FocusSafeButton>
           </div>
         </div>
       </div>