fix: narrow telegram chat display names

[dutchiono]

Summary

• centralize Telegram chat display-name selection behind a narrowed helper
• avoid accessing chat fields that are not present on every Telegraf chat variant

Why

Windows Milady installs were surfacing a TypeScript build failure in this plugin because ctx.chat is a union and the code accessed title, first_name, and username without narrowing.

Validation

• bun run build

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[lalalune]

Closing as superseded — current alpha already narrows ctx.chat and resolves the chat display name via title / first_name / username checks in src/service.ts (lines 68-85) and src/messageManager.ts (lines 764-770). The Windows TS build error this PR addressed is resolved. Thanks @dutchiono.