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/features/pool/components/PoolSchedulingDialog.vue

@@ -334,7 +334,7 @@ const FALLBACK_PRESET_DEFS: PoolPresetMeta[] = [
     label: 'Free/Team 优先',
     description: '兼容旧配置：优先消耗 Free、Team 或两者',
     evidence_hint: '依据 plan_type，保留旧 free_only/team_only/both 语义',
-    providers: ['codex', 'grok', 'kiro', 'windsurf'],
+    providers: ['codex', 'grok', 'kiro', 'windsurf', 'xai'],
     modes: [
       { value: 'free_only', label: 'Free' },
       { value: 'team_only', label: 'Team' },
@@ -347,7 +347,7 @@ const FALLBACK_PRESET_DEFS: PoolPresetMeta[] = [
     label: 'Free 优先',
     description: '优先消耗 Free 账号（依赖 plan_type）',
     evidence_hint: '依据 plan_type（Free 账号优先调度）',
-    providers: ['codex', 'grok', 'kiro', 'windsurf'],
+    providers: ['codex', 'grok', 'kiro', 'windsurf', 'xai'],
     modes: null,
     default_mode: null,
   },
@@ -356,7 +356,7 @@ const FALLBACK_PRESET_DEFS: PoolPresetMeta[] = [
     label: 'Team 优先',
     description: '优先消耗 Team 账号（依赖 plan_type）',
     evidence_hint: '依据 plan_type（Team 账号优先调度）',
-    providers: ['codex', 'grok', 'kiro', 'windsurf'],
+    providers: ['codex', 'grok', 'kiro', 'windsurf', 'xai'],
     modes: null,
     default_mode: null,
   },
@@ -365,7 +365,7 @@ const FALLBACK_PRESET_DEFS: PoolPresetMeta[] = [
     label: 'Plus 优先',
     description: '优先消耗 Plus 账号（依赖 plan_type）',
     evidence_hint: '依据 plan_type（Plus 账号优先调度）',
-    providers: ['codex', 'grok', 'kiro', 'windsurf'],
+    providers: ['codex', 'grok', 'kiro', 'windsurf', 'xai'],
     modes: null,
     default_mode: null,
   },
@@ -374,7 +374,7 @@ const FALLBACK_PRESET_DEFS: PoolPresetMeta[] = [
     label: 'Pro 优先',
     description: '优先消耗 Pro 账号（依赖 plan_type）',
     evidence_hint: '依据 plan_type（Pro 账号优先调度）',
-    providers: ['codex', 'grok', 'kiro', 'windsurf'],
+    providers: ['codex', 'grok', 'kiro', 'windsurf', 'xai'],
     modes: null,
     default_mode: null,
   },
@@ -392,7 +392,7 @@ const FALLBACK_PRESET_DEFS: PoolPresetMeta[] = [
     label: '额度刷新优先',
     description: '优先选即将刷新额度的账号',
     evidence_hint: '依据账号额度重置倒计时（next_reset / reset_seconds）',
-    providers: ['codex', 'grok', 'kiro', 'windsurf'],
+    providers: ['codex', 'grok', 'kiro', 'windsurf', 'xai'],
     default_enabled_providers: ['codex', 'windsurf'],
     modes: null,
     default_mode: null,