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/channelCells.tsx

@@ -4,6 +4,7 @@ import type { ManagedChannel } from "../lib/api";
 import { formatNumber } from "../lib/format";
 import Tooltip from "./ui/Tooltip";
 import { isFullySynced } from "./channelSync";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // The Channels-manager cells that are components in their own right (they read i18n), split out of
 // channelColumns so that file holds only the column factory and its predicates (Fast Refresh).
@@ -66,7 +67,7 @@ export function ActionsCell({
   return (
     <div className="inline-flex items-center gap-2">
       <Tooltip hint={backfillHint}>
-        <button
+        <FocusSafeButton
           onClick={!enabled ? undefined : isAdmin ? onReset : onDeep}
           disabled={!enabled}
           className={`shrink-0 transition ${
@@ -79,7 +80,7 @@ export function ActionsCell({
           aria-label={isAdmin ? t("channels.row.resetBackfill") : t("channels.row.backfillThis")}
         >
           <History className="w-4 h-4" />
-        </button>
+        </FocusSafeButton>
       </Tooltip>
       <Tooltip hint={c.hidden ? t("channels.row.hiddenHint") : t("channels.row.hideHint")}>
         <button