fix(dashboard): drop /api prefix in prod build

The dev setup uses Vite's /api → :8765 proxy with a rewrite that
strips the prefix. In prod the dashboard and FastAPI share an origin
with no rewrite, so /api/scenarios was 404-ing and the UI got stuck
on 'Loading scenarios…'. Set VITE_API_BASE="" so the built bundle
calls /scenarios, /predict, /query, /health directly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -7,8 +7,8 @@ WORKDIR /app/dashboard
 COPY dashboard/package.json dashboard/package-lock.json* ./
 RUN npm ci --no-audit --no-fund || npm install --no-audit --no-fund
 COPY dashboard/ ./
-# Hit the FastAPI at the same origin in prod, no dev proxy.
-ENV VITE_API_BASE=/api
+# Same-origin FastAPI in prod — routes are unprefixed (no dev /api rewrite).
+ENV VITE_API_BASE=""
 RUN npm run build
 
 # ---------- Stage 2: Python runtime ----------