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

internal/chatcmpl/client_test.go

@@ -0,0 +1,117 @@
+package chatcmpl
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestCreateNonStream(t *testing.T) {
+	var gotAuth, gotBody string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotAuth = r.Header.Get("Authorization")
+		data, _ := io.ReadAll(r.Body)
+		gotBody = string(data)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"id":"cmpl-1","choices":[{"index":0,"message":{"role":"assistant","content":"hi"},"finish_reason":"stop"}],"usage":{"prompt_tokens":5,"completion_tokens":2,"total_tokens":7}}`))
+	}))
+	defer upstream.Close()
+
+	client := NewClient(upstream.URL, "sk-up")
+	resp, err := client.Create(context.Background(), &Request{Model: "ds-chat", Messages: []Message{{Role: "user", Content: "hi"}}})
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	if gotAuth != "Bearer sk-up" {
+		t.Errorf("authorization = %q", gotAuth)
+	}
+	if !strings.Contains(gotBody, `"model":"ds-chat"`) {
+		t.Errorf("request body = %s", gotBody)
+	}
+	if len(resp.Choices) != 1 || resp.Choices[0].Message.Content != "hi" {
+		t.Fatalf("response = %+v", resp)
+	}
+	if resp.Usage == nil || resp.Usage.PromptTokens != 5 {
+		t.Errorf("usage = %+v", resp.Usage)
+	}
+}
+
+func TestCreateUpstreamError(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		_, _ = w.Write([]byte(`{"error":{"message":"bad key","type":"invalid_api_key"}}`))
+	}))
+	defer upstream.Close()
+
+	client := NewClient(upstream.URL, "sk-bad")
+	_, err := client.Create(context.Background(), &Request{Model: "m"})
+	var ue *UpstreamError
+	if !errors.As(err, &ue) {
+		t.Fatalf("error = %v, want *UpstreamError", err)
+	}
+	if ue.StatusCode != http.StatusUnauthorized || ue.Message != "bad key" {
+		t.Errorf("upstream error = %+v", ue)
+	}
+}
+
+func TestCreateStreamRejectsNon2xx(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusTooManyRequests)
+		_, _ = w.Write([]byte(`{"error":{"message":"slow down"}}`))
+	}))
+	defer upstream.Close()
+
+	client := NewClient(upstream.URL, "sk")
+	resp, err := client.CreateStream(context.Background(), &Request{Model: "m", Stream: true})
+	if err == nil {
+		resp.Body.Close()
+		t.Fatal("CreateStream succeeded on 429")
+	}
+}
+
+func TestStreamNext(t *testing.T) {
+	sse := strings.Join([]string{
+		`data: {"id":"1","choices":[{"index":0,"delta":{"content":"Hel"}}]}`,
+		"",
+		`: keep-alive comment`,
+		`data: {"id":"1","choices":[{"index":0,"delta":{"content":"lo"}}]}`,
+		"",
+		`data: {"id":"1","choices":[{"index":0,"delta":{},"finish_reason":"stop"}],"usage":{"prompt_tokens":3,"completion_tokens":1}}`,
+		"",
+		"data: [DONE]",
+		"",
+	}, "\n")
+	stream := NewStream(strings.NewReader(sse))
+
+	first, err := stream.Next()
+	if err != nil {
+		t.Fatalf("Next: %v", err)
+	}
+	if first.Choices[0].Delta.Content == nil || *first.Choices[0].Delta.Content != "Hel" {
+		t.Fatalf("first chunk = %+v", first)
+	}
+	second, err := stream.Next()
+	if err != nil {
+		t.Fatalf("Next: %v", err)
+	}
+	if *second.Choices[0].Delta.Content != "lo" {
+		t.Fatalf("second chunk = %+v", second)
+	}
+	third, err := stream.Next()
+	if err != nil {
+		t.Fatalf("Next: %v", err)
+	}
+	if third.Choices[0].FinishReason == nil || *third.Choices[0].FinishReason != "stop" {
+		t.Fatalf("third chunk = %+v", third)
+	}
+	if third.Usage == nil || third.Usage.CompletionTokens != 1 {
+		t.Fatalf("usage chunk = %+v", third)
+	}
+	if _, err := stream.Next(); err != io.EOF {
+		t.Fatalf("final Next err = %v, want io.EOF", err)
+	}
+}