feat(playlists): two-way (3-way) merge YouTube sync + mandatory preview (S7)

"Sync to YouTube" was a one-directional destructive push (local wins, deletes
from YouTube every item not in the local list), so items added on YouTube were
lost and a dirty pull was skipped entirely. Replace it with a real 3-way merge:
each item is classified against the last-synced baseline as a local-add, a
YouTube-add, or a genuine delete, so an add or delete on either side survives.

- Persist the ordered baseline id set (new playlists.synced_video_ids, migration
  0075); the fingerprint hash alone couldn't drive a merge. NULL baseline (fresh
  export / pre-S7 link) unions losslessly on the first sync and records one.
- Pure merge_plan(baseline, local, youtube) -> add/remove per side + merged order
  (local order, YouTube-only additions appended). merge_playlist applies the
  local side directly and the YouTube side through the failure breaker; the
  baseline only advances on full success so a partial failure re-converges.
- plan_merge/push-plan now report what each side gains and loses; the confirm
  dialog shows the two-way plan before applying (pushPlanMerge, HU+EN).
- Tests: 10 new (pure classification incl. delete-vs-stale-copy; DB apply proving
  a YouTube-add and a Siftlode-add both survive on both sides, a genuine delete
  propagates without collateral loss, no-baseline never deletes, create path).

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

@@ -35,8 +35,7 @@
   "pushTitle": "Sync to YouTube",
   "pushConfirm": "Sync",
   "pushPlanCreate": "Create a new YouTube playlist with {{count}} videos? (~{{units}} quota units; {{left}} left today.)",
-  "pushPlanUpdate": "Update on YouTube — add {{insert}}, remove {{del}}, reorder {{reorder}}? (~{{units}} quota units; {{left}} left today.)",
-  "pushDiverged": "{{count}} video(s) currently on YouTube will be removed.",
+  "pushPlanMerge": "Two-way sync — on YouTube: +{{addYt}} / −{{removeYt}}; here: +{{addLocal}} / −{{removeLocal}}. (~{{units}} quota units; {{left}} left today.)",
   "pushNoQuota": "Not enough YouTube quota left today ({{left}}) for this sync (~{{units}} needed). Try again tomorrow.",
   "pushUpToDate": "“{{name}}” is already in sync with YouTube.",
   "pushDone": "“{{name}}” synced to YouTube ✓",

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

@@ -35,8 +35,7 @@
   "pushTitle": "Szinkron YouTube-ra",
   "pushConfirm": "Szinkron",
   "pushPlanCreate": "Létrehozol egy új YouTube-listát {{count}} videóval? (~{{units}} kvótaegység; ma még {{left}} maradt.)",
-  "pushPlanUpdate": "Frissítés a YouTube-on — {{insert}} hozzáadása, {{del}} eltávolítása, {{reorder}} átrendezése? (~{{units}} kvótaegység; ma még {{left}} maradt.)",
-  "pushDiverged": "{{count}} videó, amely jelenleg a YouTube-on van, el lesz távolítva.",
+  "pushPlanMerge": "Kétirányú szinkron — YouTube-on: +{{addYt}} / −{{removeYt}}; itt: +{{addLocal}} / −{{removeLocal}}. (~{{units}} kvótaegység; ma még {{left}} maradt.)",
   "pushNoQuota": "Nincs elég YouTube-kvóta mára ({{left}}) ehhez a szinkronhoz (~{{units}} kell). Próbáld újra holnap.",
   "pushUpToDate": "A(z) „{{name}}” már szinkronban van a YouTube-bal.",
   "pushDone": "„{{name}}” szinkronizálva a YouTube-ra ✓",