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/PlexCollectionEditor.tsx

@@ -9,6 +9,7 @@ import { useScrollFade } from "../lib/useScrollFade";
 import { Button } from "./ui/Button";
 import Modal from "./ui/Modal";
 import { useConfirm } from "./confirmContext";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Admin-only "manage this movie's collections" dialog (Collections P2). Lists the library's EDITABLE
 // collections with an in/out toggle (add/remove the movie, written back to Plex), plus a field to
@@ -190,7 +191,7 @@ export default function PlexCollectionEditor({
                 key={c.id}
                 className="glass-card flex items-center gap-2 rounded-lg px-2 py-1.5 text-sm"
               >
-                <button
+                <FocusSafeButton
                   onClick={() => toggle(c.id)}
                   disabled={!!busy}
                   className={`inline-flex min-w-0 flex-1 items-center gap-2 text-left disabled:opacity-50 ${
@@ -208,14 +209,14 @@ export default function PlexCollectionEditor({
                   <span className="shrink-0 text-xs text-muted">
                     {t("plex.collEditor.count", { count: c.child_count ?? 0 })}
                   </span>
-                </button>
-                <button
+                </FocusSafeButton>
+                <FocusSafeButton
                   onClick={() => removeCollection(c.id, c.title)}
                   disabled={!!busy}
                   className="shrink-0 rounded-sm px-1.5 py-0.5 text-xs text-muted hover:text-danger disabled:opacity-50"
                 >
                   {t("plex.collEditor.delete")}
-                </button>
+                </FocusSafeButton>
               </div>
             );
           })
@@ -242,13 +243,13 @@ export default function PlexCollectionEditor({
                 <span className="shrink-0 text-xs text-muted">
                   {t("plex.collEditor.count", { count: c.child_count ?? 0 })}
                 </span>
-                <button
+                <FocusSafeButton
                   onClick={() => markEditable(c.id)}
                   disabled={!!busy}
                   className="glass-card glass-hover shrink-0 rounded-sm px-2 py-0.5 text-xs disabled:opacity-50"
                 >
                   {t("plex.collEditor.makeEditable")}
-                </button>
+                </FocusSafeButton>
               </div>
             ))}
           </div>