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/SearchBar.tsx

@@ -8,6 +8,7 @@ import {
 import { useTranslation } from "react-i18next";
 import { Search, X } from "lucide-react";
 import { useSearchFocusKey } from "../lib/keymap";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // The floating search pill in the top header. One component for every module that searches
 // (feed, Plex, channels, playlists): a glass rounded box with a search icon, the input, a
@@ -100,7 +101,7 @@ export default function SearchBar({
           </button>
         )}
         {onGo && (
-          <button
+          <FocusSafeButton
             onClick={fire}
             disabled={goDisabled}
             title={goTitle ?? goLabel ?? t("header.goTip")}
@@ -110,7 +111,7 @@ export default function SearchBar({
             <span className={goIcon ? "hidden md:inline" : undefined}>
               {goLabel ?? t("header.go")}
             </span>
-          </button>
+          </FocusSafeButton>
         )}
       </div>
       {below}