Refactor GrampsWeb subpath handling: move all patching to nginx sub_filter (STI-98)

Replace fragile container-side sed/JS patching with comprehensive nginx sub_filter
rules. The container now only handles admin user creation on startup. All SPA route
prefixing is done in nginx via a History API interceptor injected into HTML responses.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

compose.dev.yml

@@ -197,11 +197,36 @@ services:
       - GRAMPSWEB_ADMIN_EMAIL=admin@localhost
       - GRAMPSWEB_ADMIN_PASSWORD=gramps_dev_password
       - GRAMPSWEB_TREE=Stiftung
-      - GRAMPSWEB_BASE_URL=/
+      - GRAMPSWEB_BASE_URL=http://localhost:18090
       - GRAMPSWEB_CELERY_CONFIG__broker_url=redis://redis:6379/0
       - GRAMPSWEB_CELERY_CONFIG__result_backend=redis://redis:6379/0
       - GRAMPSWEB_RATELIMIT_STORAGE_URI=redis://redis:6379/1
       - GRAMPSWEB_NEW_DB_BACKEND=sqlite
+    command:
+      - sh
+      - -c
+      - |
+        echo "[grampsweb] Ensuring admin user exists ..."
+        python3 << 'PYEOF' 2>&1 | grep -v Gtk
+        from gramps_webapi.app import create_app
+        from gramps_webapi.auth import add_user, get_number_users, ROLE_OWNER
+        import os
+        email = os.environ.get('GRAMPSWEB_ADMIN_EMAIL', '')
+        pw = os.environ.get('GRAMPSWEB_ADMIN_PASSWORD', '')
+        if email and pw:
+            app = create_app()
+            with app.app_context():
+                if get_number_users() == 0:
+                    add_user(name='Admin', email=email, password=pw, role=ROLE_OWNER)
+                    print('[grampsweb] Admin user created')
+                else:
+                    print('[grampsweb] Users already exist, skipping')
+        else:
+            print('[grampsweb] No admin credentials configured, skipping')
+        PYEOF
+        exec gunicorn -w $${GUNICORN_NUM_WORKERS:-4} -b 0.0.0.0:5000 \
+          gramps_webapi.wsgi:app --timeout $${GUNICORN_TIMEOUT:-120} \
+          --limit-request-line 8190
     volumes:
       - gramps_data_dev:/app/data
     depends_on: