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

content/posts/setup-wsl/index.md

@@ -36,7 +36,7 @@ wsl --install
 Go to the Microsoft store and search for a Linux distribution. I prefer
 to use the "Ubuntu" distribution.
 
-![Microsoft Store](/img/WSL/ubuntu.webp)
+![Ubuntu in the Microsoft Store](ubuntu.webp)
 
 Once installed, go ahead and run the distribution, after it finishes setting
 itself up it will ask you for a username and password. These credentials are
@@ -51,7 +51,7 @@ I use the windows terminal app to run the Ubuntu distribution, it makes
 customising the environment a lot easier. To download it head over to the
 Microsoft store and search for Windows Terminal and download it.
 
-![Microsoft Store](/img/WSL/winterm.webp)
+![Windows Terminal in the Microsoft Store](winterm.webp)
 
 When you login to WSL it will not default to your home directory. To fix that go
 to the Windows Terminal Settings look for Ubuntu or your distributions name and
@@ -61,11 +61,11 @@ change the Starting directory to
 \\wsl$\<distro>\home\<username>
 ```
 
-> Be sure to change "<distro>" to your distributions name
+> Be sure to change `<distro>` to your distributions name
 
-> Be sure to change "<username>" to your username
+> Be sure to change `<username>` to your username
 
-![winterm-settings](/img/WSL/winterm-settings.webp)
+![Windows Terminal settings](winterm-settings.webp)
 
 ## **Conclusion**