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/form.tsx

@@ -1,5 +1,6 @@
 import type { ReactNode } from "react";
 import Tooltip from "./Tooltip";
+import { FocusSafeButton } from "./FocusSafeButton";
 
 // Shared form/settings primitives, factored out of the many panels that each re-declared
 // them (Settings, Config, Stats, admin pages, Setup wizard). Visual contract unchanged.
@@ -29,8 +30,10 @@ export function Switch({
   label?: string;
   disabled?: boolean;
 }) {
+  // FocusSafeButton: a switch is disabled by its caller while the change it made is saving, and a
+  // natively disabled switch that holds the focus throws it to <body>.
   return (
-    <button
+    <FocusSafeButton
       type="button"
       role="switch"
       aria-checked={checked}
@@ -44,7 +47,7 @@ export function Switch({
           checked ? "left-4.5" : "left-0.5"
         }`}
       />
-    </button>
+    </FocusSafeButton>
   );
 }