fix(youtube): a refused YouTube action answers a 422 that says what to do

The two manual syncs (subscriptions import, playlist pull) had no YouTubeError
handler, so a quota, token or transport failure was a raw 500 and the blocking
"Server error" dialog (E9 F1). The other YouTube routes answered either the
upstream's English or a playlist-scoped "youtube_failed".

One helper, routes/_common.youtube_refusal: 422 {code: "youtube", reason} with
the reason sorted by what the user can do — quota (wait for the reset),
reconnect (the token step refused), unavailable (retry). Used by every route
that surfaces a YouTube failure; localizeDetail translates it.

"Reconnect" had no path: a grant Google revoked still read as granted, so
Settings and the wizard offered no connect button while every call failed.
An invalid_grant from the token endpoint now clears the stored grant.

frontend/src/i18n/locales/en/errors.json

@@ -91,5 +91,10 @@
     "reload": "Reload",
     "notifTitle": "Something broke",
     "notifMessage": "Unexpected error"
+  },
+  "youtube": {
+    "quota": "Today's YouTube budget for this instance is spent. It resets at midnight Pacific time — try again then.",
+    "reconnect": "Google no longer accepts this account's YouTube connection. Sign out and sign in with Google again to reconnect it.",
+    "unavailable": "YouTube didn't accept that just now. Please try again."
   }
 }

frontend/src/i18n/locales/en/playlists.json

@@ -81,7 +81,6 @@
     "name_required": "Give the playlist a name.",
     "youtube_read_scope": "Connect YouTube (read access) to sync your playlists.",
     "not_on_youtube": "This playlist isn't on YouTube.",
-    "youtube_failed": "YouTube didn't accept that just now. Please try again.",
     "watch_later_not_pushable": "The Watch later list can't be sent to YouTube.",
     "watch_later_undeletable": "The Watch later list can't be deleted.",
     "youtube_editing_off": "Enable YouTube editing in Settings first.",

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

@@ -91,5 +91,10 @@
     "reload": "Újratöltés",
     "notifTitle": "Valami elromlott",
     "notifMessage": "Váratlan hiba"
+  },
+  "youtube": {
+    "quota": "Ennek a példánynak elfogyott a mai YouTube-kerete. Csendes-óceáni idő szerint éjfélkor újraindul — akkor próbáld újra.",
+    "reconnect": "A Google már nem fogadja el ennek a fióknak a YouTube-kapcsolatát. Jelentkezz ki, majd lépj be újra Google-lel az összekötéshez.",
+    "unavailable": "A YouTube most nem fogadta el. Próbáld újra."
   }
 }

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

@@ -81,7 +81,6 @@
     "name_required": "Adj nevet a lejátszási listának.",
     "youtube_read_scope": "A listák szinkronizálásához kapcsold össze a YouTube-ot (olvasási hozzáférés).",
     "not_on_youtube": "Ez a lejátszási lista nincs fent a YouTube-on.",
-    "youtube_failed": "A YouTube most nem fogadta el. Próbáld újra.",
     "watch_later_not_pushable": "A „Megnézem később” listát nem lehet a YouTube-ra küldeni.",
     "watch_later_undeletable": "A „Megnézem később” lista nem törölhető.",
     "youtube_editing_off": "Előbb engedélyezd a YouTube-szerkesztést a Beállításokban.",