Parallelize frontend tests in Drone pipeline with 4 shards

Split the monolithic "build and test" kaniko step into a DAG:
tests run in 4 parallel shards (vitest --shard) alongside the
Docker image build, gated by a shared npm ci step. The kaniko
build now targets the named 'production' stage to skip the
in-Dockerfile test stage.

frontend/Dockerfile

@@ -30,7 +30,7 @@ COPY . .
 RUN npx vite build
 
 # Stage 4: Serve with nginx
-FROM nginx:alpine
+FROM nginx:alpine AS production
 
 # Remove default nginx static files
 RUN rm -rf /usr/share/nginx/html/*