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/ChannelDiscovery.tsx

@@ -1,10 +1,11 @@
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { UserPlus } from "lucide-react";
-import { api, HttpError, type DiscoveredChannel } from "../lib/api";
-import { accountKey } from "../lib/storage";
-import { formatViews } from "../lib/format";
+import { api, type DiscoveredChannel } from "../lib/api";
+import { accountKey, LS } from "../lib/storage";
+import { formatCountOrDash } from "../lib/format";
 import { notify } from "../lib/notifications";
+import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import Tooltip from "./Tooltip";
 import ChannelLink from "./ChannelLink";
 import DataTable, { type Column } from "./DataTable";
@@ -48,19 +49,8 @@ export default function ChannelDiscovery({
         meta: { kind: "channel-subscribed", channelId: c.id, channelName: name },
       });
     },
-    onError: (err: unknown) => {
-      // A 403 means the user hasn't granted the write scope — offer to connect instead of
-      // a vague failure (mirrors the subscriptions tab).
-      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("channels.discovery.subscribeFailed") });
-      }
-    },
+    onError: (err: unknown) =>
+      notifyYouTubeActionError(err, t("channels.discovery.subscribeFailed"), onOpenWizard),
   });
 
   // Subscribing changes the user's real YouTube account and spends a little quota — confirm
@@ -96,9 +86,7 @@ export default function ChannelDiscovery({
       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>
       ),
     },
     {
@@ -109,9 +97,7 @@ export default function ChannelDiscovery({
       sortable: true,
       sortValue: (c) => c.video_count ?? -1,
       render: (c) => (
-        <span className="text-muted tabular-nums">
-          {c.video_count != null ? formatViews(c.video_count) : "—"}
-        </span>
+        <span className="text-muted tabular-nums">{formatCountOrDash(c.video_count)}</span>
       ),
     },
     {
@@ -174,7 +160,7 @@ export default function ChannelDiscovery({
           rows={rows}
           columns={columns}
           rowKey={(c) => c.id}
-          persistKey={accountKey("siftlode.channelDiscoveryTable") ?? undefined}
+          persistKey={accountKey(LS.channelDiscoveryTable) ?? undefined}
           controlsPosition="top"
           emptyText={t("channels.discovery.empty")}
         />