fix(sync): an empty answer the remote's own count contradicts is not believed, one level down

E32 review round 1 (B1, B2, B3, B8).

- Plex watch reconcile: a section the server SAYS is empty (totalSize 0, nothing listed) is a
  complete listing. It read as "cut short" forever, so one empty library stopped un-watches from
  ever syncing. A listing with no total at all stays incomplete (the client now returns None for
  it instead of 0), and a whole scan that listed nothing still clears nothing. Tests now cover
  completeness across two sections, both orders.
- Plex collections: no members from a collection whose listing counts some (childCount) keeps
  the membership we hold, in the full sync and in the targeted resync after a write.
- YouTube playlists: no items for a playlist whose listing counts some (itemCount) keeps the
  mirror in the bulk pull, and is refused as retryable in "reset to YouTube" and in the push
  plan/merge, where it read as "YouTube deleted them all". The rule sits beside vanished().
- A playlist deleted on YouTube answered "reconnect your account"; it now says it is gone
  (errors.youtube.gone, HU+EN).
- The pure 3-way merge moved to sync/playlist_merge.py; playlists.py is back under 800 lines and
  off the size list.

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

@@ -115,7 +115,8 @@
   "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."
+    "unavailable": "YouTube didn't accept that just now. Please try again.",
+    "gone": "This playlist no longer exists on YouTube"
   },
   "refused": {
     "account_no_access": "That account no longer has access",

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

@@ -115,7 +115,8 @@
   "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."
+    "unavailable": "A YouTube most nem fogadta el. Próbáld újra.",
+    "gone": "Ez a lejátszási lista már nem létezik a YouTube-on"
   },
   "refused": {
     "account_no_access": "Ennek a fióknak már nincs hozzáférése",