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/VideoEditor.tsx

@@ -12,6 +12,7 @@ import { inputCls, btnCls } from "./ui/form";
 import { Button } from "./ui/Button";
 import { failureReason, notifyActionError } from "../lib/actionErrors";
 import { markReported } from "../lib/api/core";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // mm:ss.d timecode <-> seconds.
 function tc(s: number): string {
@@ -617,7 +618,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
         <div className="flex flex-wrap items-center gap-2 text-sm">
           <span className="text-muted">{t("editor.output.label")}:</span>
           {(["separate", "join"] as const).map((m) => (
-            <button
+            <FocusSafeButton
               key={m}
               onClick={() => setOutput(m)}
               disabled={m === "join" && kept.length < 2}
@@ -629,7 +630,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
               )}
             >
               {t(`editor.output.${m}`)}
-            </button>
+            </FocusSafeButton>
           ))}
           <div className="flex-1" />
           <span className="text-xs text-muted">