fix(plex): round-3 deep-review follow-ups

Third review round, scoped to the round-2 fix commit. The production ternary was
confirmed correct; two sibling-surface findings:

- The sidebar CollectionPicker's clear-all still wrote collectionTitles:{} while the
  chip-row remove (fixed in round 2) wrote undefined — a divergence that leaves the
  {}-vs-absent cache split on the picker path. Collapse an empty map to undefined there
  too, so both no-collection routes key-match the pristine state.
- The queryKeys lang test was near-tautological (a variadic-spread factory can't drop a
  passed arg), and nothing asserted the CALL SITE keys carry lang. Add a real guard in
  PlexDiscover.dom.test.tsx that inspects the registered query keys for scope+lang, and
  reword the factory test's comment to say it only shape-pins the segment pass-through.

Gate: tsc/lint/knip/prettier clean, 413 vitest.

frontend/src/components/PlexSidebar.tsx

@@ -413,7 +413,7 @@ function CollectionPicker({
   values: string[];
   titles: Record<string, string> | undefined;
   mode: "any" | "all";
-  onChange: (values: string[], titles: Record<string, string>) => void;
+  onChange: (values: string[], titles: Record<string, string> | undefined) => void;
   onModeChange: (mode: "any" | "all") => void;
 }) {
   const { t } = useTranslation();
@@ -451,10 +451,12 @@ function CollectionPicker({
       matchMode={mode === "all" ? "and" : "or"}
       onMatchModeChange={(m) => onModeChange(m === "and" ? "all" : "any")}
       onChange={(next) => {
-        // Carry only the selected ids' titles forward (drop titles for de-selected ones).
+        // Carry only the selected ids' titles forward (drop titles for de-selected ones). Collapse an
+        // empty map to undefined so a clear-all key-matches the pristine state (an empty {} still hashes
+        // differently from an absent key), mirroring the chip-row remove handler.
         const nextTitles: Record<string, string> = {};
         for (const id of next) nextTitles[id] = labelMap[id] ?? titles?.[id] ?? id;
-        onChange(next, nextTitles);
+        onChange(next, next.length ? nextTitles : undefined);
       }}
     />
   );