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) : "—";
+}