Reapply "Add missing admin action files and navigation links"

This reverts commit 4d515bc400.

src/actions/guestbook.ts

@@ -25,7 +25,9 @@ export async function postGuestbook(formData: FormData): Promise<void> {
   const profileId = Number(formData.get("profileId"));
   if (!Number.isInteger(profileId) || profileId <= 0) return;
 
-  const message = String(formData.get("message") ?? "").trim().slice(0, MESSAGE_MAX);
+  const message = String(formData.get("message") ?? "")
+    .trim()
+    .slice(0, MESSAGE_MAX);
   if (!message) return;
 
   // Block filtered/AI-flagged content before it touches the DB (fail-open).