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/i18n/locales/en/channels.json

@@ -1,6 +1,5 @@
 {
   "intro": "Set a channel's <0>priority</0> to push its videos up when you sort by “Channel priority”, attach your own <1>tags</1> to filter the feed, or <2>hide</2> a channel to drop it from the feed without unsubscribing.",
-  "filterPlaceholder": "Filter channels…",
   "syncSubscriptions": "Read subscriptions from YouTube",
   "syncSubscriptionsHint": "Re-import your subscription list from YouTube — adds channels you've newly followed and drops ones you've unfollowed. The videos themselves keep syncing automatically in the background; this does not re-fetch them.",
   "backfillEverything": "Backfill everything",
@@ -35,9 +34,7 @@
   "tags": {
     "yourTags": "Your tags",
     "manage": "Manage tags",
-    "yourTagsHint": "Your personal labels. Attach them to channels below, then filter the feed by tag from the sidebar. (Separate from the automatic language/topic tags.)",
-    "newTag": "new tag",
-    "createTag": "Create tag"
+    "yourTagsHint": "Your personal labels. Attach them to channels below, then filter the feed by tag from the sidebar. (Separate from the automatic language/topic tags.)"
   },
   "loading": "Loading channels…",
   "empty": "No channels.",
@@ -74,8 +71,6 @@
     "quotaLeftHint": "Shared YouTube API budget left today (resets midnight US Pacific)."
   },
   "row": {
-    "stored": "{{formatted}} stored",
-    "subs": "{{formatted}} subs",
     "openOnYouTube": "Open on YouTube",
     "editTags": "Edit tags",
     "filterFeedByTag": "Filter the feed by “{{name}}”",
@@ -98,8 +93,6 @@
     "fullHistoryComing": "full history coming",
     "queuedRequestedHint": "Full history requested — this channel's whole back-catalog will backfill as the shared quota allows. Click to cancel your request.",
     "queuedByOtherHint": "Another subscriber already requested this channel's full history, so its whole back-catalog is on its way to everyone — nothing to do here.",
-    "getFullHistory": "get full history",
-    "getFullHistoryHint": "Only recent uploads so far. Click to request this channel's full back-catalog (older videos + complete search).",
     "hiddenHint": "Hidden — this channel's videos are kept out of your feed. Click to show them again.",
     "hideHint": "Hide this channel's videos from your feed. It stays subscribed; this doesn't unsubscribe on YouTube.",
     "unhide": "Unhide",