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/lib/format.ts

@@ -101,3 +101,8 @@ export function formatViews(n: number | null): string {
   if (n >= 1e3) return `${(n / 1e3).toFixed(1).replace(/\.0$/, "")}K`;
   return String(n);
 }
+
+// A count for a right-aligned table cell: the humanized number, or an em-dash when unknown (null).
+export function formatCountOrDash(n: number | null | undefined): string {
+  return n != null ? formatViews(n) : "—";
+}

frontend/src/lib/storage.ts

@@ -16,6 +16,8 @@ export const LS = {
   perfMode: "siftlode.perfMode",
   channelFilter: "siftlode.channelFilter",
   channelsView: "siftlode.channelsView",
+  channelsTable: "siftlode.channelsTable",
+  channelDiscoveryTable: "siftlode.channelDiscoveryTable",
   settingsTab: "siftlode.settingsTab",
   statsTab: "siftlode.statsTab",
   adminUsersTab: "siftlode.adminUsersTab",

frontend/src/lib/youtubeErrors.ts

@@ -0,0 +1,25 @@
+import { HttpError } from "./api";
+import { notify } from "./notifications";
+import i18n from "../i18n";
+
+// A YouTube-gated action (subscribe, sync, backfill, unsubscribe) that 403s means the user hasn't
+// granted the write scope — surface a "Connect" affordance instead of a vague failure. Pass
+// `onConnect` to offer the connect-wizard button (callers that have no wizard handle, e.g. the
+// channel page, just get the message). `fallbackMessage` is the already-translated non-403 error.
+export function notifyYouTubeActionError(
+  err: unknown,
+  fallbackMessage: string,
+  onConnect?: () => void,
+): void {
+  if (err instanceof HttpError && err.status === 403) {
+    notify({
+      level: "error",
+      message: i18n.t("channels.notify.needYouTube"),
+      action: onConnect
+        ? { label: i18n.t("channels.notify.connect"), onClick: onConnect }
+        : undefined,
+    });
+  } else {
+    notify({ level: "error", message: fallbackMessage });
+  }
+}