feat(cairn): add cairn, a guest-first directory page

[MorganKryze]

What this is: a new app.

What changed: adds Apps/cairn/, one docker-compose.yml plus icon.svg,
thumbnail.png and three screenshots. It touches nothing else in the tree.

What cairn is: the directory page for the people you host services for. You
hand it to your family, your club or your members. One YAML file in, a static
multilingual page out, with no account to create, no database and no outbound
requests. Each service can show a live status pill pulled from Gatus or Uptime
Kuma, and cairn serves its own legal and privacy pages, which associations and
small hosts need.

I'm its author. Tell me if anything below is in the wrong shape and I'll fix it.

Why it fits here: I read the 166 app folders under Apps/ and found no start
page or service directory, so cairn overlaps with nothing you already carry.

How I validated it, on a fresh clone with the app in place:

• validate_compose.py --app-path cairn: 0 errors, 0 warnings
• docker compose config -q: passes
• ./scripts/build_dist.sh: completes, 167 apps, dist/apps/org.icewhale.cairn/
  generated with meta.json, the seven meta.{locale}.json files and all assets
• dist/apps/org.icewhale.cairn/docker-compose.yml: still carries read_only,
  cap_drop: ALL, no-new-privileges and the read-only /config bind

Notes for review:

• The image is FROM scratch, two files and 4.4 MB to pull on amd64, 4.0 on
  arm64. It writes nothing at runtime, hence read_only with every capability
  dropped. The image has no shell, so the healthcheck is the binary probing
  itself: /cairn -healthcheck.
• No environment variables at all. cairn is configured by files, and it needs
  no secret to run. The one credential it can want is optional, for status pills
  against an authenticated monitoring API, and cairn reads it from a file path
  set in the YAML.
• The first run works with an empty /config: cairn serves a getting-started
  page explaining what to do. Drop a services.yaml in and it reloads within
  seconds, no restart. That is what tips.before_install says.
• I used org.icewhale.cairn for the id to match the convention I saw on the
  other apps. Happy to change it if you'd rather namespace third-party
  submissions differently.
• Metadata is localized into eight languages: en_US, fr_FR, de_DE, es_ES,
  it_IT, nl_NL, pt_PT and sv_SE. Those are the eight languages cairn's own
  interface ships, so the store card speaks what the product speaks. Each
  translation follows the register of cairn's string table: formal in German,
  Spanish, Dutch and Portuguese, informal in Italian, European rather than
  Brazilian Portuguese.

Links:

• Source: https://github.com/MorganKryze/cairn
• Live demo: https://cairn.libresoftware.cloud
• Licence: GPL-3.0

Thanks for keeping the store open to outside submissions.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud