chore(channels-ui): Phase 2 #3 frontend cleanup — dedup 403 handler, count cell, LS keys, dead i18n

- Extract notifyYouTubeActionError (new lib/youtubeErrors.ts): the "403 → Connect
  your YouTube account, else fallback toast" logic was duplicated in Channels.tsx
  and ChannelDiscovery.tsx. (Separate file, not lib/notifications, to avoid the
  api ↔ notifications import cycle.)
- Extract format.formatCountOrDash(): the `n != null ? formatViews(n) : "—"` count
  cell was repeated across the subs/videos columns in Channels + ChannelDiscovery.
- Register channelsTable / channelDiscoveryTable in storage.ts LS instead of bare
  "siftlode.*" string literals.
- Delete 7 dead channels.json keys (filterPlaceholder, tags.newTag/createTag,
  row.stored/subs/getFullHistory/getFullHistoryHint) from en/hu/de — verified 0
  refs (createTag matches were the unrelated api.createTag method, not the key).

Behavior-neutral. tsc green, knip no new unused, localdev boots.

frontend/src/components/Channels.tsx

@@ -16,10 +16,11 @@ import {
   UserMinus,
   X,
 } from "lucide-react";
-import { api, HttpError, type ManagedChannel, type Tag } from "../lib/api";
-import { accountKey } from "../lib/storage";
+import { api, type ManagedChannel, type Tag } from "../lib/api";
+import { notifyYouTubeActionError } from "../lib/youtubeErrors";
+import { accountKey, LS } from "../lib/storage";
 import { useDismiss } from "../lib/useDismiss";
-import { formatEta, formatTotalHours, formatViews, relativeTime } from "../lib/format";
+import { formatCountOrDash, formatEta, formatTotalHours, relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import DataTable, { type Column } from "./DataTable";
@@ -73,19 +74,6 @@ export default function Channels({
   const qc = useQueryClient();
   const confirm = useConfirm();
 
-  // 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 });
@@ -160,7 +148,7 @@ export default function Channels({
       invalidate();
       notify({ level: "success", message: t("channels.notify.synced", { count: r.subscriptions ?? 0 }) });
     },
-    onError: (e) => notifyActionError(e, "channels.notify.syncFailed"),
+    onError: (e) => notifyYouTubeActionError(e, t("channels.notify.syncFailed"), onOpenWizard),
   });
   const unsubscribe = useMutation({
     mutationFn: (v: { id: string; name: string }) => api.unsubscribeChannel(v.id),
@@ -169,7 +157,7 @@ export default function Channels({
       qc.invalidateQueries({ queryKey: ["my-status"] });
       notify({ level: "success", message: t("channels.notify.unsubscribed", { name: v.name }) });
     },
-    onError: (e) => notifyActionError(e, "channels.notify.unsubscribeFailed"),
+    onError: (e) => notifyYouTubeActionError(e, t("channels.notify.unsubscribeFailed"), onOpenWizard),
   });
   const resetBackfill = useMutation({
     mutationFn: (v: { id: string; name: string }) => api.resetChannelBackfill(v.id),
@@ -178,7 +166,7 @@ export default function Channels({
       qc.invalidateQueries({ queryKey: ["my-status"] });
       notify({ level: "success", message: t("channels.notify.resetDone", { name: v.name }) });
     },
-    onError: (e) => notifyActionError(e, "channels.notify.resetFailed"),
+    onError: (e) => notifyYouTubeActionError(e, t("channels.notify.resetFailed"), onOpenWizard),
   });
   const deepAll = useMutation({
     mutationFn: () => api.deepAll(true),
@@ -190,7 +178,7 @@ export default function Channels({
         message: t("channels.notify.fullHistoryRequested", { count: r.updated ?? 0 }),
       });
     },
-    onError: (e) => notifyActionError(e, "channels.notify.fullHistoryFailed"),
+    onError: (e) => notifyYouTubeActionError(e, t("channels.notify.fullHistoryFailed"), onOpenWizard),
   });
 
   // Channel-name search + tag-chip filtering, applied client-side over the status-filtered list
@@ -294,9 +282,7 @@ export default function Channels({
       sortable: true,
       sortValue: (c) => c.subscriber_count ?? -1,
       render: (c) => (
-        <span className="text-muted tabular-nums">
-          {c.subscriber_count != null ? formatViews(c.subscriber_count) : "—"}
-        </span>
+        <span className="text-muted tabular-nums">{formatCountOrDash(c.subscriber_count)}</span>
       ),
     },
     {
@@ -570,7 +556,7 @@ export default function Channels({
           rows={visibleChannels}
           columns={columns}
           rowKey={(c) => c.id}
-          persistKey={accountKey("siftlode.channelsTable") ?? undefined}
+          persistKey={accountKey(LS.channelsTable) ?? undefined}
           controlsPosition="top"
           controlsLeading={statusChips}
           rowClassName={(c) => (c.hidden ? "opacity-60" : "")}