fix: replace sing-box with mihomo for egress probe (xhttp support)

mihomo (Clash Meta) natively supports xhttp and all Clash transports.
Build minimal mihomo YAML config directly from the proxy node instead
of converting to sing-box JSON format.

Dockerfile

@@ -18,11 +18,11 @@ RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /sub-store .
 # ─── Stage 3: Runtime ───
 FROM alpine:3.21
 RUN apk add --no-cache ca-certificates tzdata && \
-    wget -qO /tmp/sing-box.tar.gz "https://github.com/SagerNet/sing-box/releases/download/v1.13.14/sing-box-1.13.14-linux-amd64-musl.tar.gz" && \
-    tar xzf /tmp/sing-box.tar.gz -C /tmp && \
-    mv /tmp/sing-box-*/sing-box /usr/local/bin/sing-box && \
-    chmod +x /usr/local/bin/sing-box && \
-    rm -rf /tmp/sing-box*
+    wget -qO /tmp/mihomo.gz "https://github.com/MetaCubeX/mihomo/releases/download/v1.19.29/mihomo-linux-amd64-v1.19.29.gz" && \
+    gunzip /tmp/mihomo.gz && \
+    mv /tmp/mihomo /usr/local/bin/mihomo && \
+    chmod +x /usr/local/bin/mihomo && \
+    rm -rf /tmp/mihomo*
 WORKDIR /app
 COPY --from=backend-builder /sub-store /app/sub-store
 COPY --from=frontend-builder /frontend/dist /app/frontend/dist