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/i18n/locales/en/channels.json

@@ -6,6 +6,7 @@
   "tabs": {
     "subscribed": "Subscriptions",
     "discovery": "Discover from playlists",
+    "hidden": "Hidden",
     "blocked": "Blocked"
   },
   "layoutLabel": "Layout",
@@ -40,8 +41,7 @@
   "filters": {
     "all": "All",
     "needsFull": "Needs full history",
-    "fullySynced": "Fully synced",
-    "hidden": "Hidden"
+    "fullySynced": "Fully synced"
   },
   "tags": {
     "yourTags": "Your tags",
@@ -139,5 +139,11 @@
     "title": "Blocked channels",
     "hint": "Their videos are hidden from your feed, Library, and live search.",
     "empty": "No blocked channels. Block one from its channel page to hide all its videos without unsubscribing."
+  },
+  "hidden": {
+    "title": "Hidden channels",
+    "hint": "Their videos are kept out of your personal feed, but still appear in the shared Library.",
+    "empty": "No hidden channels. Hide a subscription (the eye icon on its row) to keep its uploads out of your personal feed without unsubscribing.",
+    "unhide": "Unhide"
   }
 }

frontend/src/i18n/locales/hu/channels.json

@@ -6,6 +6,7 @@
   "tabs": {
     "subscribed": "Feliratkozások",
     "discovery": "Felfedezés a lejátszási listákból",
+    "hidden": "Elrejtett",
     "blocked": "Letiltott"
   },
   "layoutLabel": "Nézet",
@@ -40,8 +41,7 @@
   "filters": {
     "all": "Mind",
     "needsFull": "Hiányos előzmény",
-    "fullySynced": "Teljesen szinkronizálva",
-    "hidden": "Elrejtett"
+    "fullySynced": "Teljesen szinkronizálva"
   },
   "tags": {
     "yourTags": "Címkéid",
@@ -139,5 +139,11 @@
     "title": "Tiltott csatornák",
     "hint": "A videóik el vannak rejtve a feededből, a Library-ből és a YouTube-keresésből.",
     "empty": "Nincs tiltott csatorna. Egy csatorna oldaláról tilthatsz le egyet, hogy minden videóját elrejtsd leiratkozás nélkül."
+  },
+  "hidden": {
+    "title": "Elrejtett csatornák",
+    "hint": "A videóik kimaradnak a személyes feededből, de a megosztott Library-ben továbbra is megjelennek.",
+    "empty": "Nincs elrejtett csatorna. Egy feliratkozást a sorában (a szem ikonnal) rejthetsz el, hogy kimaradjon a személyes feededből leiratkozás nélkül.",
+    "unhide": "Felfedés"
   }
 }