feat(providers): add xAI provider with device code OAuth

Add a separate `xai` provider type for xAI Grok CLI subscription accounts.
It is independent of the existing `grok` provider, which reverse-proxies
grok.com with browser cookies; behavior of `grok` is unchanged.

Account binding uses the xAI device code flow, so no local callback
listener is needed and headless deployments can bind accounts. Refresh
tokens can also be imported individually or in batches, and are rotated
on refresh.

OAuth requests default to the cli-chat-proxy Responses API; API keys and
compact stay on api.x.ai. Explicit custom gateways are preserved. Only
`openai:responses` and `openai:responses:compact` are exposed; Chat,
Claude and Gemini clients reach the provider through Aether's existing
cross-format conversion rather than new native endpoints.

Upstream Responses payloads are sanitized for what xAI actually rejects:
`previous_response_id` and `metadata.user_id` are dropped, hosted
`tool_choice` is rewritten, `web_search` is restored for converted
clients, `image_generation` is stripped on older Grok conversation
models, unsupported reasoning effort is removed, and requested
`reasoning.encrypted_content` is preserved with a replay policy keyed on
the configured provider type rather than the model name.

Quota refresh reads /user and /billing?format=credits and stores a
structured usage snapshot; a prepaid balance keeps an account selectable
after the weekly allowance is exhausted. API-key accounts skip the
subscription billing surface. The admin UI shows remaining weekly quota
as a labeled bar in the provider drawer and the pool list.

Co-Authored-By: Claude Opus 5 <[email protected]>

frontend/src/api/endpoints/provider_oauth.ts

@@ -376,7 +376,7 @@ function jsonValueContainsAgentIdentity(value: unknown): boolean {
 export interface DeviceAuthorizeRequest {
   start_url?: string
   region?: string
-  auth_type?: 'builder_id' | 'identity_center' | 'google' | 'github' | 'browser'
+  auth_type?: 'builder_id' | 'identity_center' | 'google' | 'github' | 'browser' | 'device'
   login_option?: 'google' | 'github' | 'default'
   redirect_uri?: string
   proxy_node_id?: string

frontend/src/api/endpoints/types/provider.ts

@@ -462,6 +462,23 @@ export interface GrokUpstreamMetadata {
   account_user_id?: string | null
 }
 
+export interface XaiUpstreamMetadata {
+  updated_at?: number
+  subscription_title?: string
+  usage_percentage?: number
+  remaining_percentage?: number
+  usage_label?: string
+  usage_limit?: number
+  current_usage?: number
+  remaining?: number
+  next_reset_at?: number
+  prepaid_balance?: number
+  on_demand_cap?: number
+  on_demand_used?: number
+  on_demand_remaining?: number
+  period_type?: string
+}
+
 export interface GeminiCliTierMetadata {
   id?: string | null
   tierType?: string | null
@@ -520,6 +537,7 @@ export interface UpstreamMetadata {
   chatgpt_web?: ChatGPTWebUpstreamMetadata
   grok?: GrokUpstreamMetadata
   gemini_cli?: GeminiCliUpstreamMetadata
+  xai?: XaiUpstreamMetadata
 }
 
 // 按格式的健康度数据
@@ -758,7 +776,7 @@ export interface HealthRelatedMonitorResponse {
   related_providers: HealthRelatedMonitor[]
 }
 
-export type ProviderType = 'custom' | 'claude_code' | 'codex' | 'chatgpt_web' | 'gemini_cli' | 'antigravity' | 'kiro' | 'grok' | 'windsurf' | 'vertex_ai'
+export type ProviderType = 'custom' | 'claude_code' | 'codex' | 'chatgpt_web' | 'gemini_cli' | 'antigravity' | 'kiro' | 'grok' | 'xai' | 'windsurf' | 'vertex_ai'
 
 export interface ClaudeCodeAdvancedConfig {
   // 会话数量控制：null/undefined 表示不限制