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/i18n/messages.ts

@@ -2146,7 +2146,10 @@ const legacyExactEnglishMessages: Record<string, string> = {
   '账号不可用': 'Account unavailable',
   '日额度': 'Daily quota',
   '周额度': 'Weekly quota',
+  '月额度': 'Monthly quota',
   '剩余额度': 'Remaining quota',
+  '预付额度': 'Prepaid credits',
+  '按需额度': 'On-demand credits',
   '点击编辑优先级': 'Edit priority',
   '点击编辑倍率': 'Edit multiplier',
   '同步失败': 'Sync failed',