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/PersonalViewsModal.tsx

@@ -24,6 +24,7 @@ import { useActiveFilters } from "../lib/useActiveFilters";
 import { buildPlexActiveFilters } from "./usePlexActiveFilters";
 import { useUndoable } from "../lib/useUndoable";
 import UndoToolbar from "./ui/UndoToolbar";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Master–detail manager for saved ("personal") views: the left column lists the views (drag to
 // reorder) plus a "save the current filters" input; the right column shows the selected view's name
@@ -227,7 +228,7 @@ export default function PersonalViewsModal() {
                 onBlur={() => void onRename()}
                 className="min-w-0 flex-1 rounded-lg border border-border bg-surface px-3 py-2 text-sm font-medium focus:border-accent focus:outline-hidden focus:ring-2 focus:ring-accent/30"
               />
-              <button
+              <FocusSafeButton
                 onClick={() => void onRename()}
                 disabled={!nameChanged}
                 title={t("views.rename")}
@@ -235,7 +236,7 @@ export default function PersonalViewsModal() {
                 className="inline-flex shrink-0 items-center rounded-lg border border-border p-2 text-muted transition enabled:hover:border-accent enabled:hover:text-accent disabled:opacity-30"
               >
                 <Check className="h-4 w-4" />
-              </button>
+              </FocusSafeButton>
             </div>
             <div>
               <div className="mb-1.5 flex items-center justify-between gap-2">
@@ -283,7 +284,7 @@ export default function PersonalViewsModal() {
               >
                 {t("views.apply")}
               </Button>
-              <button
+              <FocusSafeButton
                 onClick={async () => {
                   const saved = await saveCurrent(selected.id);
                   // Re-seed the draft to what we just stored (the live filters) so the chip row
@@ -298,7 +299,7 @@ export default function PersonalViewsModal() {
               >
                 <Save className="h-3.5 w-3.5" />
                 {t("views.saveCurrent")}
-              </button>
+              </FocusSafeButton>
               <button
                 onClick={() => void toggleDefault(selected)}
                 title={t("views.default")}