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/components/Playlists.tsx

@@ -362,7 +362,15 @@ export default function Playlists() {
     setPushing(true);
     try {
       const plan = await api.playlistPushPlan(selectedId);
-      if (plan.action === "update" && !plan.to_insert && !plan.to_delete && !plan.to_reorder) {
+      const nothingToDo =
+        plan.action === "update" &&
+        !plan.add_yt &&
+        !plan.add_local &&
+        !plan.remove_yt &&
+        !plan.remove_local &&
+        !plan.to_reorder &&
+        !plan.rename;
+      if (nothingToDo) {
         notify({ level: "info", message: t("playlists.pushUpToDate", { name: plName(detail) }) });
         return;
       }
@@ -376,21 +384,21 @@ export default function Playlists() {
         });
         return;
       }
-      let message =
+      const message =
         plan.action === "create"
           ? t("playlists.pushPlanCreate", {
-              count: plan.to_insert,
+              count: plan.add_yt,
               units: plan.units_estimate,
               left: plan.remaining_today,
             })
-          : t("playlists.pushPlanUpdate", {
-              insert: plan.to_insert,
-              del: plan.to_delete,
-              reorder: plan.to_reorder,
+          : t("playlists.pushPlanMerge", {
+              addYt: plan.add_yt,
+              addLocal: plan.add_local,
+              removeYt: plan.remove_yt,
+              removeLocal: plan.remove_local,
               units: plan.units_estimate,
               left: plan.remaining_today,
             });
-      if (plan.yt_extra > 0) message += " " + t("playlists.pushDiverged", { count: plan.yt_extra });
       const ok = await confirm({
         title: t("playlists.pushTitle"),
         message,