fix(landing-page): drop trailing slash from preview iframe URLs (#2790)

The detail-page interactive preview iframe pointed at
`/skills/<slug>/example.html/` and `/templates/<slug>/preview.html/`
with trailing slashes. Cloudflare Pages 308-redirects those URLs to
the extension-stripped form, but with the trailing slash present it
fails to map back to the published `out/skills/<slug>/example.html`
file and SPA-falls-back to the homepage. Result: every preview iframe
in production rendered the homepage instead of the skill or
live-artifact preview.

Verified against the deployed site after the #2679 release:

  - /skills/deck-guizang-editorial/example.html       → 4942 bytes (real preview)
  - /skills/deck-guizang-editorial/example.html/      → 163377 bytes (homepage SPA fallback)
  - /skills/deck-guizang-editorial/example            → 4942 bytes (real preview)
  - /skills/deck-guizang-editorial/example/           → 4942 bytes (real preview)

Drop the trailing slash from all six iframe `src` and "Open in new
tab" `href` attributes in `pages/skills/[slug]/index.astro` and
`pages/templates/[slug]/index.astro`, plus the inline comment that
documented the URL shape.

Co-authored-by: Joey-nexu <joeylee12629@gmail.com>

apps/landing-page/app/pages/skills/[slug]/index.astro

@@ -257,7 +257,7 @@ const jsonLd = [
           Click-to-expand interactive preview. The thumb itself is the
           summary of a `<details>` element — clicking the image opens
           the live iframe, replacing the thumb with the canonical
-          `<slug>/example.html/` rendered inside a sandboxed frame. A
+          `<slug>/example.html` rendered inside a sandboxed frame. A
           hover overlay on the thumb hints "Click for live preview ↗"
           so the affordance is discoverable. The iframe loads lazily,
           only fetching once the user actually expands.
@@ -275,7 +275,7 @@ const jsonLd = [
           </summary>
           <div class="detail-preview-frame-wrap">
             <iframe
-              src={`/skills/${skill.slug}/example.html/`}
+              src={`/skills/${skill.slug}/example.html`}
               title={`${skill.name} interactive preview`}
               loading="lazy"
               sandbox="allow-scripts allow-same-origin"
@@ -283,7 +283,7 @@ const jsonLd = [
             />
             <a
               class="detail-preview-popout"
-              href={`/skills/${skill.slug}/example.html/`}
+              href={`/skills/${skill.slug}/example.html`}
               target="_blank"
               rel="noopener"
               aria-label="Open preview in new tab"

apps/landing-page/app/pages/templates/[slug]/index.astro

@@ -258,8 +258,8 @@ const jsonLd = {
             <iframe
               src={
                 template.origin === 'live-artifact'
-                  ? `/templates/${template.slug}/preview.html/`
-                  : `/skills/${template.slug}/example.html/`
+                  ? `/templates/${template.slug}/preview.html`
+                  : `/skills/${template.slug}/example.html`
               }
               title={`${template.name} interactive preview`}
               loading="lazy"
@@ -270,8 +270,8 @@ const jsonLd = {
               class="detail-preview-popout"
               href={
                 template.origin === 'live-artifact'
-                  ? `/templates/${template.slug}/preview.html/`
-                  : `/skills/${template.slug}/example.html/`
+                  ? `/templates/${template.slug}/preview.html`
+                  : `/skills/${template.slug}/example.html`
               }
               target="_blank"
               rel="noopener"