Close the two remaining /gift funnel gaps

Add a hero image (rendered from the redesigned PDF's own cover) so the
opt-in page shows the actual resource instead of pure text, and make
the post-submit message PDF-specific with a working /gift-thanks
fallback link instead of a generic dead-end "check your inbox" line.

Both are opt-in per-page params (hero_image, success_message) so the
real drip-course page's rendering is unchanged.

content/gift/index.md

@@ -7,9 +7,15 @@ eec_slug: "three-steps-find-your-right-readers"
 # Optional italic line under the title, for handling an objection up front.
 hero_subhead: "Three concrete steps to find the readers who actually want your stories — free PDF, no fluff, sent straight to your inbox."
 
+# Optional image shown in the hero, below the subhead.
+hero_image: "/images/gift-cover.png"
+
 # Optional label on every "Send me day one" button across the page.
 cta_text: "Send me the PDF"
 
 # Optional heading above the repeated signup form at the bottom of the page.
 repeat_cta_heading: "Ready to find your right readers?"
+
+# Optional override of the enroll form's post-submit message (rendered as markdown).
+success_message: "Check your inbox — we've sent your PDF. Can't find it? [Grab it here](/gift-thanks)."
 ---