fix(web): correct blog colors for the light workspace + thumbnails

Only the .rail sidebar is dark; the .workspace content area is on the
light (#f6f7f4) page background. The previous blog styling assumed a dark
workspace, so text was light-grey on white (unreadable) and the link
green was too light.

- Darken the global link color to #0a7d59 (readable on white); content
  links only — rail nav stays inherited.
- Repaint .blog-content (post HTML) for a light surface: dark body text,
  light code/pre, light borders.
- Blog index/post: dark titles, readable grey meta, light row borders;
  light-themed footer.
- Add post thumbnails to the /blog index from featured_image.url.

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

apps/logicsrc-web/src/components/site-shell.tsx

@@ -56,13 +56,13 @@ export function SiteShell({
             maxWidth: "72rem",
             margin: "2rem auto 0",
             padding: "1.25rem 0",
-            borderTop: "1px solid rgba(245, 247, 244, 0.12)",
+            borderTop: "1px solid #d9ded4",
             display: "flex",
             flexWrap: "wrap",
             gap: "0.75rem",
             justifyContent: "space-between",
             fontSize: "0.85rem",
-            color: "#b5beb2",
+            color: "#5b6b7a",
           }}
         >
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