CrossRelay stage 0+1: Go gateway with Anthropic<->Chat Completions relay, Tauri shell, bilingual management UI

- Go gateway (stdlib only): /v1/messages non-stream + SSE, /v1/messages/count_tokens,
  management API /api/* (providers CRUD, apply/restore, settings, backups, logs)
- Protocol conversion: Anthropic Messages <-> Chat Completions with a streaming
  state machine keyed by content block index (text, tool_use, parallel calls)
- ~/.claude/settings.json guarded read/write: timestamped backups, managed env
  keys only, one-click restore
- Tauri 2 shell: spawns the gateway sidecar, parses the random port from stdout,
  stdin pipe watchdog prevents orphans, kills the child on exit
- React frontend: Dashboard/Providers/Settings, zh-CN + en-US, TanStack Query
- Tests: Go table-driven + httptest integration (all green), vitest + RTL

cmd/gateway/main.go

@@ -0,0 +1,116 @@
+// Command gateway runs the CrossRelay local gateway: it serves the
+// Anthropic Messages API for Claude Code and the management API for the
+// CrossRelay desktop app.
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"os/signal"
+	"path/filepath"
+	"syscall"
+	"time"
+
+	"gitea.rutua.cn/kate522/CrossRelay/internal/config"
+	"gitea.rutua.cn/kate522/CrossRelay/internal/gateway"
+)
+
+// watchParentShutdown exits the gateway when the parent process goes
+// away. The Tauri shell spawns the gateway with a piped stdin and sets
+// CROSSRELAY_WATCH_PARENT=1; when the app exits, crashes, or is killed,
+// the OS closes the pipe and the gateway shuts down instead of lingering
+// as an orphan.
+func watchParentShutdown(httpSrv *http.Server, statePath string, done chan struct{}) {
+	if os.Getenv("CROSSRELAY_WATCH_PARENT") != "1" {
+		return
+	}
+	go func() {
+		buf := make([]byte, 512)
+		for {
+			if _, err := os.Stdin.Read(buf); err != nil {
+				break
+			}
+		}
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		_ = httpSrv.Shutdown(ctx)
+		_ = os.Remove(statePath)
+		close(done)
+	}()
+}
+
+func main() {
+	log.SetFlags(log.LstdFlags | log.Lmsgprefix)
+	log.SetPrefix("crossrelay: ")
+
+	store, err := config.Load()
+	if err != nil {
+		log.Fatalf("load config: %v", err)
+	}
+
+	addr := "127.0.0.1:0"
+	cfg := store.Config()
+	if cfg.Gateway.PortMode == config.PortModeFixed && cfg.Gateway.FixedPort > 0 {
+		addr = fmt.Sprintf("127.0.0.1:%d", cfg.Gateway.FixedPort)
+	}
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "CROSSRELAY_LISTEN_FAILED %s\n", err)
+		os.Exit(1)
+	}
+	port := ln.Addr().(*net.TCPAddr).Port
+
+	srv := gateway.New(store, port)
+	httpSrv := &http.Server{Handler: srv.Handler()}
+
+	statePath := filepath.Join(config.Dir(), "state.json")
+	if err := writeState(statePath, port); err != nil {
+		log.Printf("write state file: %v", err)
+	}
+
+	// The desktop app parses this line to learn the actual port.
+	fmt.Printf("CROSSRELAY_LISTENING 127.0.0.1:%d\n", port)
+
+	shutdownDone := make(chan struct{})
+	watchParentShutdown(httpSrv, statePath, shutdownDone)
+	go func() {
+		sig := make(chan os.Signal, 1)
+		signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)
+		<-sig
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		_ = httpSrv.Shutdown(ctx)
+		_ = os.Remove(statePath)
+		close(shutdownDone)
+	}()
+
+	if err := httpSrv.Serve(ln); err != nil && err != http.ErrServerClosed {
+		fmt.Fprintf(os.Stderr, "CROSSRELAY_SERVE_FAILED %s\n", err)
+		_ = os.Remove(statePath)
+		os.Exit(1)
+	}
+	<-shutdownDone
+}
+
+// writeState records the running gateway for diagnostics.
+func writeState(path string, port int) error {
+	state := map[string]any{
+		"pid":        os.Getpid(),
+		"port":       port,
+		"started_at": time.Now().UTC().Format(time.RFC3339),
+		"version":    gateway.Version,
+	}
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return err
+	}
+	return os.WriteFile(path, data, 0o600)
+}