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/ui/Pager.tsx

@@ -2,6 +2,7 @@ import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { ChevronLeft, ChevronRight } from "lucide-react";
 import { hasPagerContent, pageCount } from "./pagerContent";
+import { FocusSafeButton } from "./FocusSafeButton";
 
 // The Prev / page-input / Next + "N per page" cluster, lifted out of DataTable so a non-table view
 // (the channel manager's card layout) can paginate the same way. Controlled: the caller owns page +
@@ -58,14 +59,14 @@ export default function Pager({
     <div className="flex items-center gap-3 flex-wrap">
       {totalPages > 1 && (
         <div className="flex items-center gap-2">
-          <button
+          <FocusSafeButton
             onClick={() => goToPage(Math.max(0, safePage - 1))}
             disabled={safePage === 0}
             className="glass-card glass-hover flex items-center gap-1 px-3 py-1.5 rounded-lg disabled:opacity-40 transition"
           >
             <ChevronLeft className="w-4 h-4" />
             {t("datatable.pager.prev")}
-          </button>
+          </FocusSafeButton>
           <span className="text-muted flex items-center gap-1.5">
             {t("datatable.pager.pageLabel")}
             <input
@@ -81,14 +82,14 @@ export default function Pager({
             />
             {t("datatable.pager.ofTotal", { total: totalPages })}
           </span>
-          <button
+          <FocusSafeButton
             onClick={() => goToPage(Math.min(totalPages - 1, safePage + 1))}
             disabled={safePage >= totalPages - 1}
             className="glass-card glass-hover flex items-center gap-1 px-3 py-1.5 rounded-lg disabled:opacity-40 transition"
           >
             {t("datatable.pager.next")}
             <ChevronRight className="w-4 h-4" />
-          </button>
+          </FocusSafeButton>
         </div>
       )}
       {total > 0 && (