feat(labs): merge markers, edit their aliases and pick a marker on review

The marker list offers a merge with the server's preview: what moves, what
is converted, which names become aliases, and a refusal when a unit cannot
be converted; it says a merge cannot be undone. The marker page shows and
edits the other spellings and follows a merged marker to its target. Scan
and document review name the matched marker or offer its suggestions and a
new marker; the choice is remembered. The biomarkerMerge feature flag is
on.

src/app/api/auth/me/route.ts

@@ -89,7 +89,7 @@ const SERVER_FEATURES = {
   // form work; until then a client must not send it (contract §0.6).
   cumulativeHourBuckets: false,
   syncTriggerBackfill: false,
-  biomarkerMerge: false,
+  biomarkerMerge: true,
   lifeCircumstances: false,
 } as const;