Prepend Undo(Like) instead of restructuring Undo(Follow)

Reviewer noted that the chapter 22 Undo handler in the tutorial
read very differently from the chapter 11 baseline because the
Follow body had been wrapped in an if-block to share the listener
with the new Like branch. Restructure the example so the
Undo(Like) branch is prepended ahead of the original
`if (!(object instanceof Follow)) return;` guard, leaving the
Follow body untouched. The new branch `return`s on success so
the guard never fires for Like objects.

Addresses fedify-dev/fedify#731 (comment)

Assisted-by: Claude Code:claude-opus-4-7

Author: dahlia

server/federation.ts

@@ -207,30 +207,6 @@ federation
   })
   .on(Undo, async (ctx, undo) => {
     const object = await undo.getObject();
-    // Undo(Follow): a remote follower has unfollowed alice; drop
-    // the followers row keyed by the original follower's URI.
-    if (object instanceof Follow) {
-      if (undo.actorId == null || object.objectId == null) return;
-      const target = ctx.parseUri(object.objectId);
-      if (target?.type !== "actor") return;
-      const localUser = (
-        await db
-          .select()
-          .from(users)
-          .where(eq(users.username, target.identifier))
-          .limit(1)
-      )[0];
-      if (localUser === undefined) return;
-      await db
-        .delete(followers)
-        .where(
-          and(
-            eq(followers.followingId, localUser.id),
-            eq(followers.actorUri, undo.actorId.href),
-          ),
-        );
-      return;
-    }
     // Undo(Like): a remote actor has retracted a like.  Match by
     // the embedded Like's id (when the peer echoes the original)
     // or by (actor, object) since Pixelfed sometimes mints a
@@ -259,7 +235,30 @@ federation
       const matcher =
         conditions.length === 1 ? conditions[0] : or(...conditions);
       await db.delete(likes).where(matcher);
+      return;
     }
+    // Undo(Follow): a remote follower has unfollowed alice; drop
+    // the followers row keyed by the original follower's URI.
+    if (!(object instanceof Follow)) return;
+    if (undo.actorId == null || object.objectId == null) return;
+    const target = ctx.parseUri(object.objectId);
+    if (target?.type !== "actor") return;
+    const localUser = (
+      await db
+        .select()
+        .from(users)
+        .where(eq(users.username, target.identifier))
+        .limit(1)
+    )[0];
+    if (localUser === undefined) return;
+    await db
+      .delete(followers)
+      .where(
+        and(
+          eq(followers.followingId, localUser.id),
+          eq(followers.actorUri, undo.actorId.href),
+        ),
+      );
   })
   .on(Accept, async (_ctx, accept) => {
     // The remote server has accepted alice's outbound Follow.