feat(channels): add a Hidden tab; move hidden channels out of Subscriptions (R13 S2)

Hidden subscriptions get their own tab (mirroring Blocked): a flat list with an
Unhide action, instead of dimmed rows scattered through the Subscriptions table.
- ChannelsView += "hidden"; the subscribed table now EXCLUDES hidden channels,
  so hiding a channel (the eye button) moves it to the Hidden tab and unhiding
  (Eye) returns it — both via the existing patch mutation.
- The "Hidden" top-level status chip is dropped (ChannelStatusFilter loses
  "hidden"); the status row is now purely sync-status (all/needs_full/fully_synced).
- Bilingual i18n (en/hu): channels.tabs.hidden + a channels.hidden block; the now
  unused channels.filters.hidden removed.

Hidden = out of the personal (Mine) feed only, still in the shared Library — a
softer exclusion than Blocked (hidden everywhere). Gate green: tsc/eslint-0/
prettier/vitest 106. Self-E2E'd: hide → appears in Hidden tab → unhide → back,
all /api 200.

frontend/src/components/ChannelsProvider.tsx

@@ -27,18 +27,13 @@ import type { ChannelStatusFilter, ChannelsView } from "./Channels";
 // per-account persisted (localStorage); search / focus / the reset token are ephemeral. An account
 // switch does a full reload, so init-from-storage is all the hydration the persisted ones need.
 
-const STATUS_FILTERS: readonly ChannelStatusFilter[] = [
-  "needs_full",
-  "fully_synced",
-  "hidden",
-  "all",
-];
+const STATUS_FILTERS: readonly ChannelStatusFilter[] = ["needs_full", "fully_synced", "all"];
 
 function clampStatus(v: string | null): ChannelStatusFilter {
   return STATUS_FILTERS.includes(v as ChannelStatusFilter) ? (v as ChannelStatusFilter) : "all";
 }
 function clampView(v: string | null): ChannelsView {
-  return v === "discovery" ? "discovery" : v === "blocked" ? "blocked" : "subscribed";
+  return v === "discovery" || v === "blocked" || v === "hidden" ? v : "subscribed";
 }
 
 type ChannelsState = {