feat: support textbox themes on say dialogue

[A-kirami]

概要

为 say 对话增加文本框主题能力，使文本框样式默认跟随说话人，并支持通过 -theme=... 在单句对话上覆盖样式。

改动内容

• 为 stage state 新增 textboxTheme
• 提取 resolveDialogDisplayState，集中处理对话显示态解析
• say 默认根据 speaker 推导文本框主题
• 支持通过 -theme=... 覆盖当前句的文本框主题
• clear 默认同时清空显示名和文本框主题
• 在文本框根节点暴露 data-textbox-theme，供模板样式使用
• 更新 TextPreview 的 props 以适配新增的主题状态
• 为对话显示态解析补充测试

使用示例

anon: sometext;
anon: sometext -theme=system;

.TextBox_Container[data-textbox-theme='anon'] .TextBox_Background {
  background: #FF8899;
}

[gemini-code-assist]

Code Review

This pull request introduces a textboxTheme property to the stage state, enabling dynamic dialogue box themes based on the speaker or explicit overrides. Key changes include the addition of a resolveDialogDisplayState utility function with unit tests, and updates to the UI components to support the new theme attribute. A review comment suggested simplifying the logic in the new utility function using more idiomatic TypeScript operators.