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/plexPlayerControls.tsx

@@ -1,5 +1,6 @@
 import { type ReactNode } from "react";
 import { RotateCcw } from "lucide-react";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // ±10s skip glyph: an outline circular arrow with a "10" in the centre — the YouTube/Plex-native
 // muscle-memory shape. Deliberately distinct from the episode-level SkipBack/SkipForward (double
@@ -139,14 +140,14 @@ export function Ctrl({
     align === "start" ? "left-0" : align === "end" ? "right-0" : "left-1/2 -translate-x-1/2";
   return (
     <div className="relative flex group/ctrl">
-      <button
+      <FocusSafeButton
         onClick={onClick}
         disabled={disabled}
         aria-label={label}
         className="p-1 hover:text-accent disabled:opacity-30"
       >
         {children}
-      </button>
+      </FocusSafeButton>
       <span
         className={`pointer-events-none absolute bottom-full ${pos} mb-2 whitespace-nowrap rounded-sm bg-black/90 px-2 py-1 text-2xs text-white opacity-0 transition group-hover/ctrl:opacity-100`}
       >