Fix: upgrade to Node 22 for Vite 8 compatibility

Dockerfile

@@ -1,7 +1,7 @@
 # ============================================================
 # Stage 1 — Build the React frontend
 # ============================================================
-FROM node:18-slim AS frontend-build
+FROM node:22-slim AS frontend-build
 
 WORKDIR /app/frontend
 
@@ -61,10 +61,10 @@ HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
 
 # ── Start with Gunicorn (production WSGI server) ──────────
 CMD ["gunicorn", \
-     "--bind", "0.0.0.0:7860", \
-     "--workers", "4", \
-     "--timeout", "120", \
-     "--access-logfile", "-", \
-     "--error-logfile", "-", \
-     "--chdir", "/app/backend", \
-     "app:app"]
+    "--bind", "0.0.0.0:7860", \
+    "--workers", "4", \
+    "--timeout", "120", \
+    "--access-logfile", "-", \
+    "--error-logfile", "-", \
+    "--chdir", "/app/backend", \
+    "app:app"]