Add multi-language support with instant switching

[Copilot]

Implements i18n system with English and Simplified Chinese. Users can switch languages in settings with immediate UI updates—no restart required.

Core Infrastructure

• src/i18n/mod.rs: Language enum, TOML translation loader, compile-time embedding via include_str!()
• assets/locales/{en,zh-CN}.toml: Externalized UI strings (52 translations each)
• Settings: Added language field, persisted to config

UI Integration

All user-facing strings now use i18n.t(key):

• Settings panel with language selector
• Main board tooltips (Clear All, Pin/Unpin)
• Tray menu items
• Search placeholder

Language selection updates search placeholder immediately for instant feedback:

button.on_click(cx.listener(move |board, _, window, cx| {
    board.selected_language = index;
    // Instant preview
    if let Ok(temp_i18n) = I18n::new(lang_copy) {
        board.search_input.update(cx, |input, cx| {
            input.set_placeholder(temp_i18n.t("search_placeholder"), window, cx);
        });
    }
    cx.notify();
}))

Extensibility

New languages require: (1) add TOML file, (2) add enum variant, (3) update load_language() match arm.

Translation failures log warnings but gracefully fallback to [Missing: key] format.

Original prompt

添加一个多语言的功能。用户可以在设置中选择具体的语种。把各个语言的字符串以配置文件的形式存在。用户可以一键全局切换。

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