Drop PaperMod theme, use minimal self-contained templates

PaperMod's opengraph.html calls partial with a redundant "partials/"
prefix that the production Hugo version (0.154.5, from the
hugomods/hugo:exts build image) doesn't tolerate, breaking the build.
Rather than patch a vendored theme, replaced it with plain, dependency-
free layouts (baseof/index/single/list) that render the actual homepage
content directly. Verified building successfully against the exact
Hugo image/version the deploy pipeline uses.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

layouts/_default/single.html

@@ -0,0 +1,4 @@
+{{ define "main" }}
+<h1>{{ .Title }}</h1>
+{{ .Content }}
+{{ end }}