Enable mock data mode in Docker build

Without a backend API server in the Docker deployment, the app was
trying to reach localhost:4000 causing a 30s timeout on page load
(content store) and a connection error on Nostr login (auth store).

Setting VITE_USE_MOCK_DATA=true makes both use the built-in mock
data and local Nostr relay instead.

Co-authored-by: Cursor <cursoragent@cursor.com>

Dockerfile

@@ -16,6 +16,10 @@ COPY . .
 # via the /relay nginx proxy at runtime (instead of hardcoding localhost)
 ENV VITE_NOSTR_RELAYS=""
 
+# Enable mock data mode — no backend API server in this deployment,
+# so auth and content use built-in mock/local data instead of timing out
+ENV VITE_USE_MOCK_DATA=true
+
 # Build the application
 RUN npm run build