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/SyncStatus.tsx

@@ -8,6 +8,7 @@ import { notifyActionError } from "../lib/actionErrors";
 import { formatViews } from "../lib/format";
 import { PopoverSurface } from "./ui/AnchoredPopover";
 import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Per-user status (not the global catalog): shows the number of videos available to *this*
 // user, how many of *their* channels are still being fetched, and how many lack full
@@ -97,14 +98,14 @@ export default function SyncStatus({
   );
 
   const pauseBtn = isAdmin && (data.paused || syncing > 0 || notFull > 0) && (
-    <button
+    <FocusSafeButton
       onClick={() => toggle.mutate()}
       disabled={toggle.isPending}
       title={data.paused ? t("header.sync.resume") : t("header.sync.pause")}
       className="p-1 rounded-md hover:bg-card hover:text-fg transition"
     >
       {data.paused ? <Play className="w-3.5 h-3.5" /> : <Pause className="w-3.5 h-3.5" />}
-    </button>
+    </FocusSafeButton>
   );
 
   // Chip: the headline count rides in the header; the state, the history link and pause live in a