Migrate omsorgapp to browser SPA, add Docker/CI build setup

- omsorgapp: drop Electron, run as a plain Vite/React browser app; refresh
  token moves to an HttpOnly cookie (omsorgCore), CORS added for the new
  browser origin, document download/preview switched to Blob-based browser
  APIs.
- Add Dockerfiles for omsorgCore, omsorgapp, and omsorgWeb, a docker-compose.yml
  wiring Postgres/MySQL/all three apps together, and a Gitea Actions workflow
  that builds and pushes images to the repo's container registry on push to
  main and on version tags.

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

omsorgapp/src/modules/auth/ForgotPasswordUsernamePage.jsx

@@ -24,6 +24,8 @@ export default function ForgotPasswordUsernamePage({ onNext, onBackToLogin }) {
 
     if (result.status === "sent") {
       onNext(username);
+    } else if (result.status === "email_unavailable") {
+      setError("Der E-Mail-Versand ist gerade nicht verfügbar. Bitte später erneut versuchen oder einen Administrator kontaktieren.");
     } else {
       setCannotReset(true);
     }