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/anthropic/sse.go

@@ -0,0 +1,32 @@
+package anthropic
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+// flusherOf extracts the http.Flusher when the writer supports it.
+func flusherOf(w http.ResponseWriter) http.Flusher {
+	if f, ok := w.(http.Flusher); ok {
+		return f
+	}
+	return nil
+}
+
+// writeEvent emits one Server-Sent Event in Anthropic's format:
+// "event: <name>\ndata: <json>\n\n".
+func writeEvent(w io.Writer, f http.Flusher, name string, data any) error {
+	payload, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	if _, err := fmt.Fprintf(w, "event: %s\ndata: %s\n\n", name, payload); err != nil {
+		return err
+	}
+	if f != nil {
+		f.Flush()
+	}
+	return nil
+}