feat: hide long original text on QQ, add /source and View Original button

[WhiteFeather127]

Changes

• QQ side (Discord→QQ): When original text > 200 chars, only show translation. Original text is stored and can be retrieved by replying with /source.
• Discord side (QQ→Discord): When original text > 200 chars, only show translation with a View Original button. Clicking sends an ephemeral message with the original text.
• MessageStore: Added support for storing original text alongside message ID mappings.
• UI: Replaced PaginatorView with ToggleOriginalView for per-user ephemeral toggling.

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中文说明

• QQ 侧（Discord→QQ）：当原文超过 200 字时，只发送翻译内容。原文存储在本地，QQ 用户可回复 /source 查看原文。
• Discord 侧（QQ→Discord）：当原文超过 200 字时，只显示翻译，附带 View Original 按钮。点击后以临时消息方式显示原文，并带 Close 按钮关闭。
• MessageStore：在原有的消息 ID 映射基础上，增加了原文存储功能。
• UI：将 PaginatorView 替换为 ToggleOriginalView，支持用户独享的原文切换。