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/components/usePlaylistActions.dom.test.tsx

@@ -49,7 +49,7 @@ const linked = { id: 4, name: "Mix", kind: "user", yt_playlist_id: "PL1" } as co
 describe("usePlaylistActions — deleting a list", () => {
   it("does not toast a failure the request layer already put on screen", async () => {
     vi.spyOn(globalThis, "fetch").mockResolvedValue(
-      new Response(JSON.stringify({ detail: { code: "playlist", reason: "youtube_failed" } }), {
+      new Response(JSON.stringify({ detail: { code: "youtube", reason: "unavailable" } }), {
         status: 422,
         headers: { "content-type": "application/json" },
       }),