fix(plex): address S2 /code-review findings (typeahead focus + a11y + test)

- Decouple the typeahead's dismiss from real focus: a separate `dismissed`
  flag (set on Escape/pick, cleared on typing/refocus) replaces manually
  clearing `focused`, so closing the panel no longer wedges it shut while the
  input keeps DOM focus — typing again re-opens it (verified in-browser).
- Scroll the highlighted option into view during arrow-key nav (listRef +
  data-idx + scrollIntoView), instead of leaving listRef unused.
- Wire the input as a combobox for screen readers: role=combobox,
  aria-autocomplete/expanded/controls/activedescendant + per-option ids.
- Drop the redundant client-side .slice (the request already caps at limit=10).
- SearchBar gains an `inputProps` passthrough for the combobox ARIA.
- Add PlexSearchBox.dom.test.tsx (4 tests): per-scope pill options, suggestion
  render + keyboard pick→jumpTo, and the Escape→retype reopen regression.

Reviewer's scrollbar-blur concern was a false positive — the listbox container
is not focusable, so a mousedown on it never blurs the input.

frontend/src/components/SearchBar.tsx

@@ -1,4 +1,9 @@
-import { type FocusEventHandler, type KeyboardEventHandler, type ReactNode } from "react";
+import {
+  type FocusEventHandler,
+  type InputHTMLAttributes,
+  type KeyboardEventHandler,
+  type ReactNode,
+} from "react";
 import { useTranslation } from "react-i18next";
 import { Search, X } from "lucide-react";
 
@@ -26,6 +31,7 @@ export default function SearchBar({
   onFocus,
   onBlur,
   onKeyDown,
+  inputProps,
 }: {
   value: string;
   onChange: (q: string) => void;
@@ -41,6 +47,9 @@ export default function SearchBar({
   onFocus?: FocusEventHandler<HTMLInputElement>;
   onBlur?: FocusEventHandler<HTMLInputElement>;
   onKeyDown?: KeyboardEventHandler<HTMLInputElement>;
+  // Extra attributes spread onto the <input> (e.g. Plex's combobox ARIA). Spread FIRST so the
+  // component's own value/handlers always win.
+  inputProps?: InputHTMLAttributes<HTMLInputElement>;
 }) {
   const { t } = useTranslation();
   const fire = () => {
@@ -51,6 +60,7 @@ export default function SearchBar({
       <div className="h-11 pl-3 pr-2 rounded-2xl glass-menu flex items-center gap-2 focus-within:outline-2 focus-within:outline-accent/60 focus-within:outline-offset-2">
         {leading ?? <Search className="w-4 h-4 shrink-0 text-muted" />}
         <input
+          {...inputProps}
           value={value}
           onChange={(e) => onChange(e.target.value)}
           onFocus={onFocus}