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/UndoToolbar.tsx

@@ -2,6 +2,7 @@ import { useEffect } from "react";
 import { useTranslation } from "react-i18next";
 import { Redo2, Undo2 } from "lucide-react";
 import { dispatch, useKeyScope } from "../../lib/keymap";
+import { FocusSafeButton } from "./FocusSafeButton";
 
 // Reusable undo/redo buttons with Ctrl/Cmd+Z (undo) and Ctrl/Cmd+Y or Ctrl/Cmd+Shift+Z
 // (redo) shortcuts. Pairs with useUndoable but takes plain props, so it works with any
@@ -47,7 +48,7 @@ export default function UndoToolbar({
     "p-1.5 rounded-lg border border-border text-muted enabled:hover:text-fg enabled:hover:border-accent disabled:opacity-30 transition";
   return (
     <div className="inline-flex items-center gap-1">
-      <button
+      <FocusSafeButton
         onClick={onUndo}
         disabled={!canUndo}
         title={t("common.undo")}
@@ -55,8 +56,8 @@ export default function UndoToolbar({
         className={btn}
       >
         <Undo2 className="w-4 h-4" />
-      </button>
-      <button
+      </FocusSafeButton>
+      <FocusSafeButton
         onClick={onRedo}
         disabled={!canRedo}
         title={t("common.redo")}
@@ -64,7 +65,7 @@ export default function UndoToolbar({
         className={btn}
       >
         <Redo2 className="w-4 h-4" />
-      </button>
+      </FocusSafeButton>
     </div>
   );
 }