docs(readme): add the Trendshift badge

[PerryLink]

This adds the Trendshift badge to the existing badge block, next to npm / CI / stars / license / dshfind.

Why this repository has one. Trendshift tracks it and it has ranked on the daily list:

• omdsh-dev/DSH-better-sidebar has been tracked since 2026-08-10, and 2026-08-14 was its best day: +591 stars, which put it at chore: 移除旧字段 dshClient（只适配新 dsh 的 dsh.client 声明） #7 on the TypeScript daily list (and 标题：conversation.chat.turnTail 拦截注册报错：slot 未声明（应改用 ctx.slots） #15 across all languages).
• The per-day record is public: https://trendshift.io/repositories/151476
• It can also be checked without trusting me: GET https://trendshift.io/api/search?q=dsh-better-sidebar returns repository_id: 151476.

What the badge does. It is not a static image — the endpoint renders the rank of the day the repository last ranked, and it returns 404 when the repository is not on a list, so the image simply disappears rather than showing a stale claim. The ?language=TypeScript variant is the TypeScript list; dropping that query parameter shows the all-languages rank instead. I used the language variant because the repository is TypeScript and the surrounding badge block already carries language/version context.

Scope. One line added in README.md, nothing else. No files moved, no text reworded, no dependency touched. Fully reversible by deleting that line.

If you would rather not carry a badge that reflects a rank that can change over time, close this and no harm done — the data link above stays valid either way.

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One optional thing while you are here (ignore it freely). This package gets ~32,198 npm installs a week and its README never asks anyone for anything. The comparable plugins convert far better: dsh-web-all about 76 stars per 1,000 installs, modlens about 43, this one about 14. We do not lack users; we lack the moment where a user is told a star is a thing they can give.

If you want it, this is the whole change (both languages, and it says plainly that nothing is unlocked by it):

## ⭐ 如果它帮到了你

这个插件是 DSH 插件家族的一员。如果你在用，**给个 star** —— 它不会解锁任何功能，但会让下一个人在搜索里更容易找到它。

*English:* if it is useful, **a star helps the next person find it** — nothing is gated behind it.

Happy to add it as its own one-line commit in this PR if you prefer, or drop it entirely — the badge change above stands on its own either way.