docs: scrub siegeln.net and LAN IPs from customer-facing docs

Replace personal-infra references with brand-aligned placeholders so
the public-facing setup docs no longer leak the user's home lab.

- HOWTO.md: cameleer.siegeln.net OIDC example → cameleer.example.com;
  loader-image default in env-var table → registry.cameleer.io (matches
  the production default flipped in the previous commit); k3s service-URL
  table 192.168.50.86 → <your-cluster-host>
- ui/README.md: dev-server VITE_API_TARGET example LAN IP → host placeholder
- ui/vite.config.ts: api.cameleer.siegeln.net comment → api.cameleer.example.com

Out of scope (intentionally kept on internal infra):
- ui/package.json dev:remote and generate-api:live LAN IPs (dev-team
  convenience scripts)
- pom.xml, .npmrc, ui/Dockerfile (buildtime registry)
- deploy/* manifests (internal k3s)
- CLAUDE.md and .claude/rules/cicd.md (handled in the next commit, which
  also adds the buildtime/public split clarification)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/vite.config.ts

@@ -2,7 +2,7 @@ import { defineConfig } from 'vite';
 import react from '@vitejs/plugin-react';
 
 // Use VITE_API_TARGET to proxy to a remote server, e.g.:
-//   VITE_API_TARGET=https://api.cameleer.siegeln.net npm run dev
+//   VITE_API_TARGET=https://api.cameleer.example.com npm run dev
 const apiTarget = process.env.VITE_API_TARGET || 'http://localhost:8081';
 
 export default defineConfig({