Fix audit findings: bugs, security, perf, content

Bugs:
- setup-wsl: fix broken /img/WSL/* image paths (now bundle-relative;
  images were broken on the published post) + better alt text
- pi-docker-swarm: remove broken missing cluster.webp reference
- archetypes/posts.md: fix malformed {{ }} template so 'hugo new' works
- rename arrstack/arrstack.md -> index.md so it's a proper leaf bundle
- static/admin/config.yml: fix 'htpps' -> 'https' site_url typo
- fix silently-dropped <placeholder> text (unsafe=false dropped them):
  wrap <distro>/<username>/<user>/<version>/<ip address> etc in backticks
  across setup-wsl, securing-ssh, pi-docker-swarm, mc-server-on-docker

Security:
- render-link.html: add rel=noopener noreferrer to external target=_blank
- extend-footer.html: fix rel='me' trapped in class attr (broke Mastodon
  verification); add proper rel/target
- static/admin/index.html: pin Sveltia CMS to 0.172.4 via jsDelivr + SRI

Performance:
- hugo.toml: ignoreFiles for *.aseprite/*.bak (stop shipping source art;
  non-page files 23 -> 13)
- video shortcode: preload=metadata; image shortcode: loading=lazy decoding=async
- sws.toml: drop SPA page-fallback so missing paths return real 404 (404.html)

Content/SEO:
- normalize tag casing (HomeLab/RemoteAccess/SSH/HUGO -> lowercase),
  fix 'prodcutivity' typo, arrstack tags:[""] -> [], add welcome description
- delete stray index.md.bak

static/admin/index.html

@@ -8,7 +8,13 @@
     <!-- Include the script that enables Netlify Identity on this page. -->
   </head>
   <body>
-    <!-- Include the script that builds the page and powers Decap CMS -->
-    <script src="https://unpkg.com/@sveltia/cms/dist/sveltia-cms.js"></script>
+    <!-- Sveltia CMS, pinned to a specific version with Subresource Integrity.
+         Bump the version + regenerate the integrity hash when upgrading:
+         curl -sS https://cdn.jsdelivr.net/npm/@sveltia/cms@<ver>/dist/sveltia-cms.js \
+           | openssl dgst -sha384 -binary | openssl base64 -A -->
+    <script
+      src="https://cdn.jsdelivr.net/npm/@sveltia/cms@0.172.4/dist/sveltia-cms.js"
+      integrity="sha384-Sj4Mfbg9OjjwG2ZE/YeUYu7xbZRTXGFg/wa/nszj3KzItKuRLEmGqSpV5C9YI2Ge"
+      crossorigin="anonymous"></script>
   </body>
 </html>