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/stream.go

@@ -0,0 +1,51 @@
+package chatcmpl
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// Stream reads Server-Sent Events carrying Chat Completions chunks.
+type Stream struct {
+	scanner *bufio.Scanner
+}
+
+// NewStream wraps a reader of upstream SSE data.
+func NewStream(r io.Reader) *Stream {
+	sc := bufio.NewScanner(r)
+	sc.Buffer(make([]byte, 0, 64*1024), 16*1024*1024)
+	return &Stream{scanner: sc}
+}
+
+// Next returns the next chunk. It reports io.EOF once the upstream
+// stream is finished (either by "[DONE]" or by closing).
+func (s *Stream) Next() (*StreamChunk, error) {
+	for s.scanner.Scan() {
+		line := strings.TrimRight(s.scanner.Text(), "\r")
+		if line == "" || strings.HasPrefix(line, ":") {
+			continue
+		}
+		if !strings.HasPrefix(line, "data:") {
+			continue
+		}
+		data := strings.TrimSpace(strings.TrimPrefix(line, "data:"))
+		if data == "" {
+			continue
+		}
+		if data == "[DONE]" {
+			return nil, io.EOF
+		}
+		var chunk StreamChunk
+		if err := json.Unmarshal([]byte(data), &chunk); err != nil {
+			return nil, fmt.Errorf("decode upstream chunk: %w", err)
+		}
+		return &chunk, nil
+	}
+	if err := s.scanner.Err(); err != nil {
+		return nil, err
+	}
+	return nil, io.EOF
+}