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/types.go

@@ -0,0 +1,124 @@
+// Package chatcmpl describes the OpenAI-compatible Chat Completions
+// protocol spoken by upstream providers.
+package chatcmpl
+
+import "encoding/json"
+
+// Request is a Chat Completions request body.
+type Request struct {
+	Model         string         `json:"model"`
+	Messages      []Message      `json:"messages"`
+	MaxTokens     int            `json:"max_tokens,omitempty"`
+	Temperature   *float64       `json:"temperature,omitempty"`
+	TopP          *float64       `json:"top_p,omitempty"`
+	Stop          []string       `json:"stop,omitempty"`
+	Stream        bool           `json:"stream,omitempty"`
+	StreamOptions *StreamOptions `json:"stream_options,omitempty"`
+	Tools         []Tool         `json:"tools,omitempty"`
+	ToolChoice    any            `json:"tool_choice,omitempty"`
+}
+
+// StreamOptions toggles the usage chunk at the end of a stream.
+type StreamOptions struct {
+	IncludeUsage bool `json:"include_usage"`
+}
+
+// Message is one Chat Completions message. Content is either a string or
+// a []ContentPart.
+type Message struct {
+	Role       string     `json:"role"`
+	Content    any        `json:"content"`
+	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
+	ToolCallID string     `json:"tool_call_id,omitempty"`
+}
+
+// ContentPart is one part of a multi-modal message.
+type ContentPart struct {
+	Type     string    `json:"type"`
+	Text     string    `json:"text,omitempty"`
+	ImageURL *ImageURL `json:"image_url,omitempty"`
+}
+
+// ImageURL references an image, usually as a data URL.
+type ImageURL struct {
+	URL string `json:"url"`
+}
+
+// Tool is a function tool definition.
+type Tool struct {
+	Type     string       `json:"type"`
+	Function ToolFunction `json:"function"`
+}
+
+// ToolFunction is the payload of a function tool.
+type ToolFunction struct {
+	Name        string          `json:"name"`
+	Description string          `json:"description,omitempty"`
+	Parameters  json.RawMessage `json:"parameters,omitempty"`
+}
+
+// ToolCall is a function call requested by the model.
+type ToolCall struct {
+	ID       string           `json:"id"`
+	Type     string           `json:"type"`
+	Function ToolCallFunction `json:"function"`
+	Index    int              `json:"index,omitempty"`
+}
+
+// ToolCallFunction holds the name and the accumulated JSON arguments.
+type ToolCallFunction struct {
+	Name      string `json:"name"`
+	Arguments string `json:"arguments"`
+}
+
+// Response is a non-streaming Chat Completions response body.
+type Response struct {
+	ID      string   `json:"id"`
+	Object  string   `json:"object"`
+	Model   string   `json:"model"`
+	Choices []Choice `json:"choices"`
+	Usage   *Usage   `json:"usage,omitempty"`
+	Error   *Error   `json:"error,omitempty"`
+}
+
+// Choice is one completion choice.
+type Choice struct {
+	Index        int     `json:"index"`
+	Message      Message `json:"message"`
+	FinishReason string  `json:"finish_reason"`
+}
+
+// Usage reports token counts.
+type Usage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+// Error is an upstream error payload.
+type Error struct {
+	Message string `json:"message"`
+	Type    string `json:"type"`
+	Code    any    `json:"code,omitempty"`
+}
+
+// StreamChunk is one SSE data payload of a streaming response.
+type StreamChunk struct {
+	ID      string        `json:"id"`
+	Choices []ChunkChoice `json:"choices"`
+	Usage   *Usage        `json:"usage,omitempty"`
+}
+
+// ChunkChoice is one choice inside a stream chunk.
+type ChunkChoice struct {
+	Index        int     `json:"index"`
+	Delta        Delta   `json:"delta"`
+	FinishReason *string `json:"finish_reason"`
+}
+
+// Delta is the incremental payload of a stream chunk.
+type Delta struct {
+	Role      string     `json:"role,omitempty"`
+	Content   *string    `json:"content,omitempty"`
+	ToolCalls []ToolCall `json:"tool_calls,omitempty"`
+}