fix(youtube): a refused YouTube action answers a 422 that says what to do

The two manual syncs (subscriptions import, playlist pull) had no YouTubeError
handler, so a quota, token or transport failure was a raw 500 and the blocking
"Server error" dialog (E9 F1). The other YouTube routes answered either the
upstream's English or a playlist-scoped "youtube_failed".

One helper, routes/_common.youtube_refusal: 422 {code: "youtube", reason} with
the reason sorted by what the user can do — quota (wait for the reset),
reconnect (the token step refused), unavailable (retry). Used by every route
that surfaces a YouTube failure; localizeDetail translates it.

"Reconnect" had no path: a grant Google revoked still read as granted, so
Settings and the wizard offered no connect button while every call failed.
An invalid_grant from the token endpoint now clears the stored grant.

frontend/src/lib/api/core.test.ts

@@ -167,6 +167,23 @@ describe("req — a structured refusal reads in the user's language", () => {
       detail: localized,
     });
   });
+
+  it.each(["quota", "reconnect", "unavailable"])(
+    "says what to do about an action YouTube refused (%s)",
+    async (reason) => {
+      // Waiting for tomorrow's budget, reconnecting and retrying are different advice; the manual
+      // syncs used to answer a raw 500 here, and the other routes the upstream's English.
+      vi.spyOn(globalThis, "fetch").mockResolvedValue(
+        new Response(JSON.stringify({ detail: { code: "youtube", reason } }), {
+          status: 422,
+          headers: { "content-type": "application/json" },
+        }),
+      );
+      await expect(req("/api/sync/subscriptions", { method: "POST" })).rejects.toMatchObject({
+        detail: `errors.youtube.${reason}`,
+      });
+    },
+  );
 });
 
 describe("req — what it has already put in front of the user", () => {

frontend/src/lib/api/core.ts

@@ -129,6 +129,10 @@ function localizeDetail(d: unknown): string {
     return o.reason === "name_taken"
       ? t("plex.collection.nameTaken")
       : t("plex.collection.editFailed");
+  // An action YouTube refused (routes/_common.youtube_refusal): what the reader can do about it —
+  // wait for the budget, reconnect, or retry. An unknown reason reads as the retryable one.
+  if (o?.code === "youtube")
+    return t(`errors.youtube.${o.reason}`, t("errors.youtube.unavailable"));
   // A playlist action the server refused (Watch later can't be deleted, YouTube said no, …).
   if (o?.code === "playlist") return t(`playlists.errors.${o.reason}`, t("errors.generic"));
   // An admin action on an account or an invite that the server refused: activating an address that