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-Hant.json

@@ -179,8 +179,11 @@
         "tpmMessage": "請求過多。請稍等片刻。",
         "tpmMessageDetailed": "達到速率限制（{limit} 令牌/分鐘）。請等待 {seconds} 秒後再傳送請求。",
         "messageApi": "看來您今天的體驗次數已達上限。非常高興您玩得開心，雖然本專案由字節跳動豆包慷慨贊助，但為了確保大家都能公平使用，我們不得不對使用量做一點小小的限制。",
+        "messageApiSelfHosted": null,
         "messageToken": "看來您今天的 Token 用量已達上限。非常高興您玩得開心，雖然本專案由字節跳動豆包慷慨贊助，但為了確保大家都能公平使用，我們不得不對使用量做一點小小的限制。",
+        "messageTokenSelfHosted": null,
         "tip": "<strong>提示：</strong>您可以使用自己的 API 金鑰（點擊設定圖示）或自行託管專案來繞過這些限制。",
+        "tipSelfHosted": "<strong>提示：</strong>您可以在設定中配置自己的 API 金鑰以繼續使用服務。",
         "reset": "您的限制將在明天重設。感謝您的理解。",
         "doubaoSponsorship": "<a href=\"{link}\" target=\"_blank\" rel=\"noopener noreferrer\" class=\"underline hover:text-foreground\">點此註冊</a>可獲得每個模型 50 萬免費 Token（包括豆包、DeepSeek 和 Kimi），然後在模型設定中配置您的 API Key。",
         "configModel": "使用您的金鑰",