Add weeknote for May 16-22, 2026; center captioned figures

Convert image shortcodes to standard markdown syntax, resize images to
1200px long edge, and fix figure centering in _extra.scss (auto margins).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/css/_extra.scss

@@ -155,12 +155,15 @@
 
 // Image and figure styling
 figure {
-  margin: 2rem 0;
+  margin: 2rem auto;
+  text-align: center;
 
   img {
     display: block;
     max-width: 100%;
     height: auto;
+    margin-left: auto;
+    margin-right: auto;
     border-radius: 8px;
     box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
   }