feat: Turn off certain features of quota popup for self-hosting (#703)

* feat: Turn off certain features of quota popup for self-hosting

This commit introduces a new variable, NEXT_PUBLIC_SELFHOSTED,
that alters the behavior of the quota popup. Downstream
consumers of the application may have their own quota-checking
logic, and the front-end reacts to the 429 error by displaying
the quota popup.  In the case of a self-hosted version of the app,
it is inappropriate to ask for sponsorship or provide a
hyperlink to the public version of the tool to apply for an
increased quota. An alternative string translation is provided
with an empty message for adopter customization.

To use this feature, compile with NEXT_PUBLIC_SELFHOSTED=true
and those parts of the quota popup will be omitted.
The downstream consumer is still expected to customize
the internationalized strings for the popup content
to be appropriate to their organization on their local forks.

Signed-off-by: Bryon Nevis <bryon.nevis@intel.com>

* refactor: improve readability and provide sensible selfhosted defaults

- Extract nested ternary expressions into quotaMessage and tipHtml variables
- Combine two separate !isSelfHosted conditional blocks into one
- Replace null tipSelfHosted with meaningful default strings across all locales

---------

Signed-off-by: Bryon Nevis <bryon.nevis@intel.com>
Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

lib/i18n/dictionaries/zh.json

@@ -179,8 +179,11 @@
         "tpmMessage": "请求过多。请稍等片刻。",
         "tpmMessageDetailed": "达到速率限制（{limit} 令牌/分钟）。请等待 {seconds} 秒后再发送请求。",
         "messageApi": "看来您今天的体验次数已达上限。非常高兴您玩得开心，虽然本项目由字节跳动豆包慷慨赞助，但为了确保大家都能公平使用，我们不得不对使用量做一点小小的限制。",
+        "messageApiSelfHosted": null,
         "messageToken": "看来您今天的 Token 用量已达上限。非常高兴您玩得开心，虽然本项目由字节跳动豆包慷慨赞助，但为了确保大家都能公平使用，我们不得不对使用量做一点小小的限制。",
+        "messageTokenSelfHosted": null,
         "tip": "<strong>提示：</strong>您可以使用自己的 API 密钥（点击设置图标）或自托管项目来绕过这些限制。",
+        "tipSelfHosted": "<strong>提示：</strong>您可以在设置中配置自己的 API 密钥以继续使用服务。",
         "reset": "您的限制将在明天重置。感谢您的理解。",
         "doubaoSponsorship": "<a href=\"{link}\" target=\"_blank\" rel=\"noopener noreferrer\" class=\"underline hover:text-foreground\">点击此处注册</a>可获得每个模型 50 万免费 Token（包括豆包、DeepSeek 和 Kimi），然后在模型设置中配置您的 API Key。",
         "configModel": "使用您的密钥",