fix(security): remove unused openssh-client from Docker image

openssh-client was never used — uptop uses pure Go SSH via
charmbracelet/ssh. Removing it eliminates CVE-2026-25680,
CVE-2026-35386, CVE-2026-35387, and CVE-2026-35388.

Dockerfile

@@ -17,8 +17,7 @@ RUN --mount=type=cache,target=/go/pkg/mod \
 # --- Stage 2: Runner ---
 FROM alpine:3.23
 WORKDIR /app
-RUN apk add --no-cache ca-certificates openssh-client
-RUN apk upgrade --no-cache
+RUN apk add --no-cache ca-certificates && apk upgrade --no-cache
 RUN mkdir /data
 
 COPY --from=builder /app/uptop .