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/PlexCollectionView.tsx

@@ -13,7 +13,7 @@ import { usePref } from "../lib/usePref";
 import SortControl from "./ui/SortControl";
 import { type SortState } from "../lib/columnSort";
 import { useConfirm } from "./confirmContext";
-import { BackBtn, ActionBtn, DetailHero } from "../lib/plexDetailUi";
+import { BackBtn, ActionBtn, DetailHero } from "./plexDetailUi";
 import { PlexPosterCard } from "./plexCards";
 import { PlexPinMenu } from "./ComfortPinMenu";
 import { useContextMenu } from "./ui/useContextMenu";
@@ -21,6 +21,7 @@ import PlexLibraryPicker from "./PlexLibraryPicker";
 import { toggleWatchState } from "../lib/plexWatch";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import NavGrid from "./ui/NavGrid";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // The Collection info page (ask B): a hero (art/title/cert/year-range/count/runtime/summary) + the
 // ordered member grid, a Play-all / Shuffle over the playable leaves, and — for a NON-external,
@@ -411,7 +412,7 @@ export default function PlexCollectionView({
                       menuProps={pinMenu.triggerProps}
                     />
                     {canEdit && (
-                      <button
+                      <FocusSafeButton
                         onClick={() => doRemove(c)}
                         disabled={busy}
                         title={t("plex.collection.removeItem")}
@@ -419,7 +420,7 @@ export default function PlexCollectionView({
                         className="glass-card glass-hover absolute right-1.5 top-1.5 z-base grid h-6 w-6 place-items-center rounded-full text-fg disabled:opacity-50"
                       >
                         <X className="h-3.5 w-3.5" />
-                      </button>
+                      </FocusSafeButton>
                     )}
                   </div>
                 ))}
@@ -489,14 +490,14 @@ function AddItemPanel({
         renderAction={(c) => {
           const inColl = existing.has(c.id) || added.has(c.id);
           return (
-            <button
+            <FocusSafeButton
               onClick={() => add(c)}
               disabled={inColl}
               className="glass-card glass-hover inline-flex items-center gap-1 rounded-lg px-2.5 py-1 text-xs disabled:opacity-50"
             >
               {inColl ? <Check className="h-3.5 w-3.5" /> : <Plus className="h-3.5 w-3.5" />}
               {inColl ? t("plex.collection.added") : t("plex.collection.add")}
-            </button>
+            </FocusSafeButton>
           );
         }}
       />