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/nginx.conf

@@ -0,0 +1,12 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # SPA-Fallback: kein serverseitiges Routing (app.jsx hält activePage als lokalen State,
+    # siehe omsorgapp/CLAUDE.md) - jede Route liefert index.html, React übernimmt den Rest.
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}