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/AdminUsers.tsx

@@ -42,6 +42,7 @@ import { useAccountPersistedState } from "../lib/storage";
 import { PageToolbar } from "./ui/PageShell";
 import { useMe } from "../lib/useMe";
 import { notifyActionError, notifyKnownFailure } from "../lib/actionErrors";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Admin-only user & access management: roles, access requests (the Invite whitelist), and the
 // demo whitelist + reset. Each section is its own tab (persisted across reloads); the Access tab
@@ -728,7 +729,7 @@ function AdminInvites() {
                         : t("settings.invites.inviteHint")
                     }
                   >
-                    <button
+                    <FocusSafeButton
                       onClick={() => onInvite(i)}
                       disabled={invite.isPending}
                       className={`shrink-0 p-1 rounded-md disabled:opacity-50 transition ${
@@ -739,7 +740,7 @@ function AdminInvites() {
                       aria-label={t("settings.invites.inviteLabel", { email: i.email })}
                     >
                       <Mails className="w-3.5 h-3.5" />
-                    </button>
+                    </FocusSafeButton>
                   </Tooltip>
                 )}
                 {/* An entry with an account behind it stays read-only: deleting it would revoke that
@@ -759,14 +760,14 @@ function AdminInvites() {
                   </button>
                 ) : (
                   <Tooltip hint={t("settings.invites.deleteHint")}>
-                    <button
+                    <FocusSafeButton
                       onClick={() => onDelete(i)}
                       disabled={remove.isPending}
                       className="shrink-0 p-1 rounded-md text-muted hover:text-danger disabled:opacity-50 transition"
                       aria-label={t("settings.invites.deleteLabel", { email: i.email })}
                     >
                       <Trash2 className="w-3.5 h-3.5" />
-                    </button>
+                    </FocusSafeButton>
                   </Tooltip>
                 )}
               </div>
@@ -876,7 +877,7 @@ function AdminDemo() {
 
       <div className="mt-4 pt-3 border-t border-border">
         <Tooltip hint={t("settings.demo.resetHint")}>
-          <button
+          <FocusSafeButton
             onClick={async () => {
               if (
                 await confirm({
@@ -893,7 +894,7 @@ function AdminDemo() {
           >
             <RotateCcw className={`w-4 h-4 ${reset.isPending ? "animate-spin" : ""}`} />
             {t("settings.demo.reset")}
-          </button>
+          </FocusSafeButton>
         </Tooltip>
       </div>
     </Section>