feat(channels): clearer error when a YouTube action lacks the scope

api: HttpError now carries the server's detail. Channels sync/backfill/unsubscribe
now detect a 403 (no YouTube grant) and show a 'connect your YouTube account'
message with a Connect action that opens the onboarding wizard, instead of a vague
'failed' toast. Translated HU/EN/DE.

frontend/src/App.tsx

@@ -203,6 +203,7 @@ export default function App() {
               canWrite={meQuery.data!.can_write}
               statusFilter={channelFilter}
               setStatusFilter={setChannelFilter}
+              onOpenWizard={() => setWizardOpen(true)}
               onViewChannel={(id, name) => {
                 setFilters({ ...filters, channelId: id, channelName: name, show: "all" });
                 setPage("feed");

frontend/src/components/Channels.tsx

@@ -13,7 +13,7 @@ import {
   UserMinus,
   X,
 } from "lucide-react";
-import { api, type ManagedChannel, type Tag } from "../lib/api";
+import { api, HttpError, type ManagedChannel, type Tag } from "../lib/api";
 import { formatEta } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
@@ -33,14 +33,30 @@ export default function Channels({
   onViewChannel,
   statusFilter,
   setStatusFilter,
+  onOpenWizard,
 }: {
   canWrite: boolean;
   onViewChannel: (id: string, name: string) => void;
   statusFilter: ChannelStatusFilter;
   setStatusFilter: (f: ChannelStatusFilter) => void;
+  onOpenWizard: () => void;
 }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
+
+  // A YouTube-gated action (sync, backfill, unsubscribe) that 403s means the user hasn't
+  // granted the needed scope — surface that with a "Connect" action instead of a vague fail.
+  const notifyActionError = (err: unknown, fallbackKey: string) => {
+    if (err instanceof HttpError && err.status === 403) {
+      notify({
+        level: "error",
+        message: t("channels.notify.needYouTube"),
+        action: { label: t("channels.notify.connect"), onClick: onOpenWizard },
+      });
+    } else {
+      notify({ level: "error", message: t(fallbackKey) });
+    }
+  };
   const channelsQuery = useQuery({ queryKey: ["channels"], queryFn: api.channels });
   const tagsQuery = useQuery({ queryKey: ["tags"], queryFn: api.tags });
   const statusQuery = useQuery({ queryKey: ["my-status"], queryFn: api.myStatus });
@@ -96,7 +112,7 @@ export default function Channels({
       invalidate();
       notify({ level: "success", message: t("channels.notify.synced", { count: r.subscriptions ?? 0 }) });
     },
-    onError: () => notify({ level: "error", message: t("channels.notify.syncFailed") }),
+    onError: (e) => notifyActionError(e, "channels.notify.syncFailed"),
   });
   const createTag = useMutation({
     mutationFn: (name: string) => api.createTag({ name, category: "other" }),
@@ -116,7 +132,7 @@ export default function Channels({
       qc.invalidateQueries({ queryKey: ["my-status"] });
       notify({ level: "success", message: t("channels.notify.unsubscribed") });
     },
-    onError: () => notify({ level: "error", message: t("channels.notify.unsubscribeFailed") }),
+    onError: (e) => notifyActionError(e, "channels.notify.unsubscribeFailed"),
   });
   const deepAll = useMutation({
     mutationFn: () => api.deepAll(true),
@@ -128,7 +144,7 @@ export default function Channels({
         message: t("channels.notify.fullHistoryRequested", { count: r.updated ?? 0 }),
       });
     },
-    onError: () => notify({ level: "error", message: t("channels.notify.fullHistoryFailed") }),
+    onError: (e) => notifyActionError(e, "channels.notify.fullHistoryFailed"),
   });
 
   const channels = (channelsQuery.data ?? [])

frontend/src/i18n/locales/de/channels.json

@@ -63,7 +63,9 @@
     "unsubscribed": "Bei YouTube abbestellt",
     "unsubscribeFailed": "Abbestellen fehlgeschlagen",
     "fullHistoryRequested": "Vollständiger Verlauf für {{count}} Kanäle angefordert",
-    "fullHistoryFailed": "Vollständiger Verlauf konnte nicht angefordert werden"
+    "fullHistoryFailed": "Vollständiger Verlauf konnte nicht angefordert werden",
+    "needYouTube": "Verbinde dein YouTube-Konto, um dies zu tun.",
+    "connect": "Verbinden"
   },
   "confirmUnsubscribe": "Kanal „{{name}}“ bei YouTube abbestellen? Dies ändert dein echtes YouTube-Konto. Um ihn nur aus deinem Feed zu entfernen, blende ihn stattdessen aus."
 }

frontend/src/i18n/locales/en/channels.json

@@ -63,7 +63,9 @@
     "unsubscribed": "Unsubscribed on YouTube",
     "unsubscribeFailed": "Unsubscribe failed",
     "fullHistoryRequested": "Full history requested for {{count}} channels",
-    "fullHistoryFailed": "Couldn't request full history"
+    "fullHistoryFailed": "Couldn't request full history",
+    "needYouTube": "Connect your YouTube account to do this.",
+    "connect": "Connect"
   },
   "confirmUnsubscribe": "Unsubscribe from \"{{name}}\" on YouTube? This changes your real YouTube account. To just remove it from your feed, hide it instead."
 }

frontend/src/i18n/locales/hu/channels.json

@@ -63,7 +63,9 @@
     "unsubscribed": "Leiratkozva a YouTube-on",
     "unsubscribeFailed": "A leiratkozás sikertelen",
     "fullHistoryRequested": "Teljes előzmény kérve {{count}} csatornához",
-    "fullHistoryFailed": "Nem sikerült teljes előzményt kérni"
+    "fullHistoryFailed": "Nem sikerült teljes előzményt kérni",
+    "needYouTube": "Ehhez csatlakoztasd a YouTube-fiókod.",
+    "connect": "Csatlakoztatás"
   },
   "confirmUnsubscribe": "Leiratkozol a(z) „{{name}}” csatornáról a YouTube-on? Ez megváltoztatja a valódi YouTube-fiókodat. Ha csak a hírfolyamodból szeretnéd eltávolítani, inkább rejtsd el."
 }

frontend/src/lib/api.ts

@@ -95,9 +95,11 @@ export interface VersionInfo {
 
 class HttpError extends Error {
   status: number;
-  constructor(status: number) {
+  detail?: string; // server-provided reason (FastAPI's `detail`), when present
-    super(`HTTP ${status}`);
+  constructor(status: number, detail?: string) {
+    super(detail || `HTTP ${status}`);
     this.status = status;
+    this.detail = detail;
   }
 }
 
@@ -128,11 +130,19 @@ async function req(url: string, opts: RequestInit = {}): Promise<any> {
     throw e;
   }
   if (!r.ok) {
+    // Capture the server's reason (FastAPI returns `{ detail }`) so callers can react.
+    let detail: string | undefined;
+    try {
+      const body = await r.json();
+      if (body && typeof body.detail === "string") detail = body.detail;
+    } catch {
+      /* no JSON body */
+    }
     // Server faults are worth surfacing; 401/403/404 etc. are handled by callers.
     if (r.status >= 500) {
       notifyErrorThrottled(`Server error ${r.status}`, `${method} ${url}`);
     }
-    throw new HttpError(r.status);
+    throw new HttpError(r.status, detail);
   }
   return r.status === 204 ? null : r.json();
 }