test(fe): lock query-key factory shapes; unify nullable guard (R7 S2 review)

Code-review follow-ups on the S2 factory:
- add queryKeys.test.ts pinning every branch (bare vs param, null-segment
  preservation, single-id, variadic composite spreads) — the contract the
  epic exists to protect now has a test (suite 68 -> 73)
- unify feed/feedCount/facets on the `!== undefined` guard the rest of the
  factory already uses (behaviour-identical for the always-truthy FeedFilters,
  removes the collapse-on-null footgun)

Two findings skipped with rationale: variadic composite-key typing (heterogeneous
object/array segments make precise tuples high-effort; root centralized + shapes
now tested) and a pre-existing thread(null) no-op (unreachable in practice).

frontend/src/lib/queryKeys.ts

@@ -16,15 +16,17 @@ export const qk = {
   tags: () => ["tags"] as const,
 
   // --- feed / video ---
-  feed: (filters?: FeedFilters) => (filters ? (["feed", filters] as const) : (["feed"] as const)),
+  // Every parameterized key guards on `!== undefined` (not truthiness), so "no arg → bare prefix
+  // key" is the ONE rule across the factory. It also keeps a nullable segment intact: `q`/`id`
+  // accept null because some call sites key on nullable state (`selectedId`, the search string),
+  // and a NULL is a real key segment that must be preserved (`["playlist", null]`) rather than
+  // silently collapsed to the prefix.
+  feed: (filters?: FeedFilters) =>
+    filters !== undefined ? (["feed", filters] as const) : (["feed"] as const),
   feedCount: (filters?: FeedFilters) =>
-    filters ? (["feed-count", filters] as const) : (["feed-count"] as const),
+    filters !== undefined ? (["feed-count", filters] as const) : (["feed-count"] as const),
   facets: (filters?: FeedFilters) =>
-    filters ? (["facets", filters] as const) : (["facets"] as const),
-  // `q`/`id` accept null (not just undefined) because the call sites key on nullable state
-  // (`selectedId`, the search string): a NULL is a real key segment and must be preserved
-  // (`["playlist", null]`), while a genuinely absent arg is the bare prefix key. The guard is
-  // `!== undefined`, so only the no-arg call collapses to the prefix.
+    filters !== undefined ? (["facets", filters] as const) : (["facets"] as const),
   ytSearch: (q?: string | null, count?: number) =>
     q !== undefined ? (["yt-search", q, count] as const) : (["yt-search"] as const),
   videoDetail: (id: string) => ["video-detail", id] as const,