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/ConfigPanel.tsx

@@ -17,6 +17,7 @@ import FilterSelect from "./ui/FilterSelect";
 import { DraftSaveBar, type SaveState } from "./ui/DraftSaveBar";
 import { LS, useAccountPersistedState } from "../lib/storage";
 import { notifyActionError } from "../lib/actionErrors";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Admin Configuration page: edit DB-overridable operational settings (registry-driven from the
 // backend — adding a key there makes it appear here automatically). Edits are drafted and
@@ -205,14 +206,14 @@ export default function ConfigPanel() {
                     with no error anywhere — say so next to the fields that fix it. */}
                 {!me.email_configured && <InlineNotice>{t("config.emailOff")}</InlineNotice>}
                 <Tooltip hint={t("config.testEmailHint")}>
-                  <button
+                  <FocusSafeButton
                     onClick={() => testEmail.mutate()}
                     disabled={testEmail.isPending || dirty}
                     className="glass-card glass-hover inline-flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
                   >
                     <Send className={`w-4 h-4 ${testEmail.isPending ? "animate-pulse" : ""}`} />
                     {t("config.testEmail")}
-                  </button>
+                  </FocusSafeButton>
                 </Tooltip>
               </div>
             )}
@@ -220,14 +221,14 @@ export default function ConfigPanel() {
             {activeGroup === "plex" && (
               <div className="mt-3 pt-3 border-t border-border/60">
                 <Tooltip hint={t("config.plexTestHint")}>
-                  <button
+                  <FocusSafeButton
                     onClick={() => testPlex.mutate()}
                     disabled={testPlex.isPending || dirty}
                     className="glass-card glass-hover inline-flex items-center gap-2 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
                   >
                     <Plug className={`w-4 h-4 ${testPlex.isPending ? "animate-pulse" : ""}`} />
                     {t("config.plexTest")}
-                  </button>
+                  </FocusSafeButton>
                 </Tooltip>
                 {dirty && <p className="text-2xs text-muted mt-2">{t("config.plexTestDirty")}</p>}
                 {plexResult && (