feat(site): tighten proof, contact, and inquiry delivery

Lead with Batch on the home page, drop the product changelog, add a
domain note, and stop reporting success when SMTP fails.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/content/offer-ids.ts

@@ -0,0 +1,6 @@
+export const offerIds = ["process", "product", "collaboration"] as const;
+export type OfferId = (typeof offerIds)[number];
+
+export function isOfferId(value: string): value is OfferId {
+  return (offerIds as readonly string[]).includes(value);
+}