feat(web): manage multiple custom (OpenAI-compatible) providers in console UI

Adds first-class support for configuring more than one custom
OpenAI-compatible provider (e.g. SiliconFlow, DeepSeek, local vLLM)
and switching the active one from the web console, addressing #2838.

Backend:
- config: new `custom_providers` (list) and `custom_active_provider`
  fields, fully backward compatible with the legacy single
  `open_ai_api_base`/`model` fields (used as fallback).
- models/custom_provider.py: centralized resolver
  `resolve_custom_credentials()` returning (api_key, api_base, model),
  with active-provider selection and graceful fallback.
- chat_gpt_bot.py wired to use the resolver.
- web_channel.py: `_provider_overview` expands `custom_providers` into
  one card per provider (id `custom:<name>`, active flag, masked key);
  new POST actions `set_custom_provider`, `delete_custom_provider`,
  `set_active_custom_provider` with hermetic persistence + bridge reset.

Frontend:
- console.js: dedicated "Custom providers" section with add / edit /
  delete / set-active actions, masked-key keep-existing handling, and
  ~20 new zh/en i18n strings.
- chat.html: custom provider modal.

Tests:
- tests/test_custom_provider.py (11) - resolver/config behavior.
- tests/test_custom_provider_handlers.py (18) - write-side handlers and
  overview expansion, including duplicate-name rejection.

All 29 unit tests pass.

docs/ja/models/custom.mdx

@@ -60,3 +60,37 @@ OpenAI 互換プロトコルで接続するサードパーティのモデルサ
 ```
 /config model qwen3.5:27b
 ```
+
+## 複数のカスタムベンダーを設定する
+
+複数の OpenAI 互換サードパーティサービス（例：SiliconFlow、Qiniu）を同時に設定したい場合は、`custom_providers` リストを使用し、`custom_active_provider` で現在有効なベンダーを指定します：
+
+```json
+{
+  "bot_type": "custom",
+  "custom_active_provider": "siliconflow",
+  "custom_providers": [
+    {
+      "name": "siliconflow",
+      "api_key": "YOUR_SILICONFLOW_KEY",
+      "api_base": "https://api.siliconflow.cn/v1",
+      "model": "deepseek-ai/DeepSeek-V3"
+    },
+    {
+      "name": "qiniu",
+      "api_key": "YOUR_QINIU_KEY",
+      "api_base": "https://api.qnaigc.com/v1",
+      "model": "deepseek-v3"
+    }
+  ]
+}
+```
+
+| パラメータ | 説明 |
+| --- | --- |
+| `custom_providers` | カスタムベンダーのリスト。各項目に `name`、`api_key`、`api_base`、任意の `model` を含む |
+| `custom_active_provider` | 有効なベンダーの `name`。空の場合はリストの最初の項目が使用される |
+
+<Note>
+  `custom_providers` が空の場合、上記の単一ベンダー設定 `custom_api_key` / `custom_api_base` に自動的にフォールバックするため、既存の設定はそのまま動作します。
+</Note>