feat: add MiMo (Xiaomi) as AI provider (#887)

* feat: add MiMo (Xiaomi) as AI provider

* fix: correct MiMo default base URL, suggested models, and reasoning support

- Default base URL was the China Token Plan endpoint (tp- keys only);
  switch to https://api.xiaomimimo.com/v1 which works with standard
  pay-as-you-go sk- keys. Token Plan users can override in settings.
- Replace deprecated mimo-v2-flash suggestion with mimo-v2.5
  (v2 series was deprecated on 2026-06-30).
- Use createDeepSeek instead of createOpenAI so reasoning_content is
  passed back during multi-turn tool calls (MiMo returns 400 without
  it), matching the existing Kimi implementation.
- Add mimo to SINGLE_SYSTEM_PROVIDERS so system messages are merged.
- Fold validate-model case into the shared OpenAI-compatible group.
- Drop the Bot icon special case; models.dev serves a real xiaomi logo
  via PROVIDER_LOGO_MAP.
- Document MIMO_API_KEY/MIMO_BASE_URL in env.example and
  docs/{en,cn,ja}/ai-providers.md.

* feat: show base URL hint for MiMo in provider settings

MiMo has two endpoints tied to key type: pay-as-you-go keys (sk-...)
use the default api.xiaomimimo.com/v1, while Token Plan keys (tp-...)
require token-plan-cn.xiaomimimo.com/v1. Surface this under the Base
URL field like the existing MiniMax hint, in all four locales.

---------

Co-authored-by: mapengfei <mapengfei@srsj.com>
Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

lib/i18n/dictionaries/ja.json

@@ -34,7 +34,8 @@
         "glm": "GLM",
         "qwen": "Qwen",
         "kimi": "Kimi",
-        "qiniu": "Qiniu"
+        "qiniu": "Qiniu",
+        "mimo": "MiMo (Xiaomi)"
     },
     "chat": {
         "placeholder": "ダイアグラムを説明するか、ファイルをアップロード...",
@@ -325,6 +326,7 @@
         "baseUrlWithExample": "ベース URL（オプション、例: {example}）",
         "customEndpoint": "カスタムエンドポイント URL",
         "minimaxBaseUrlHint": "/anthropic で Anthropic 互換 API（推奨）、または /v1 で OpenAI 互換 API を使用",
+        "mimoBaseUrlHint": "デフォルトは従量課金キー（sk-...）用です。Token Plan 加入者（tp-... キー）は https://token-plan-cn.xiaomimimo.com/v1 を設定してください",
         "models": "モデル",
         "customModelId": "カスタムモデル ID...",
         "allAdded": "すべて追加済み",