fix: correct 0.15.1 blog cover image aspect ratio (16:9 -> 3:2) (#5887)

* fix: correct 0.15.1 blog cover image aspect ratio (16:9 -> 3:2)

The 0.15.1 cover shipped at 2400x1350 (16:9), wider than the site's standard
1536-wide covers, so its blog-listing card didn't line up with neighbors.
Re-cropped/scaled to 1536x1024 (matches open-design-shanghai-ai-workshop-cover.jpg);
same content (logo, version, tagline), same filenames/paths. Follow-up to #5884.

* fix: register 0.15.1 cover in blog index postImages map

The cover-file swap alone did not reach the blog index: cards render art only
when postImages[id] exists, and there was no open-design-0-15-1 entry, so the
0.15.1 card fell through to the fallback. Add the entry (points at the .webp
cover) so the index card shows the release art. Addresses @nettee's review.

---------

Co-authored-by: XingliGe <xinglige0205@gmail.com>

Author: open-design-crew[bot]

apps/landing-page/app/pages/blog/index.astro

@@ -123,6 +123,10 @@ const postImages: Record<string, { src: string; alt: string }> = {
     src: '/blog/open-design-0-14-0-inspiration-time-machine-cover.webp',
     alt: 'A warm editorial illustration of a hand-drawn idea sketch with a lightbulb becoming a calm sage-green planning workspace with a structured outline and a version-history timeline, beside soft plants and a coffee mug',
   },
+  'open-design-0-15-1': {
+    src: '/blog/open-design-0-15-1-cover.webp',
+    alt: 'A soft Fauvist oil painting of trees beside water in warm pastels, overlaid with the Open Design mark, "0.15.1", and the tagline "Sharper vision. Longer flow." with "Longer flow." highlighted in green',
+  },
   'open-design-0-15-0-cost-less-ship-faster': {
     src: '/blog/open-design-0-15-0-cost-less-ship-faster-cover.webp',
     alt: 'A warm editorial illustration of a hand-drawn deck sketch becoming a calm sage-green slide workspace shipping fast beside a timer, on a near-white paper desk with soft plants and a coffee mug',