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### What this PR does

Before this PR:
- Version was at 1.8.3

After this PR:
- Bumps version to 1.8.4
- Updates release notes with bilingual (English/Chinese) content

### Release Notes

Cherry Studio 1.8.4 - Bug Fixes and New Features

**English:**
- [API] Add knowledge base REST API endpoints
- [SelectionAssistant] Add Linux support for text selection toolbar
- [Files] Fix PDF upload failing with "invalid part type: file" error
for OpenAI-compatible providers (Moonshot, DeepSeek, Groq, etc.)

**Chinese:**
- [API] 添加知识库 REST API 端点
- [选择助手] 为文本选择工具栏添加 Linux 支持
- [文件] 修复 OpenAI 兼容提供商（Moonshot、DeepSeek、Groq 等）的 PDF 上传失败问题（"invalid
part type: file" 错误）

### Included Commits

1. fix(aiCore): remove openai-compatible providers from PDF native
support list (#13809)
2. feat(api): add knowledge base REST API endpoints (#13762)
3. feat(SelectionAssistant): Linux support (#13720)
4. fix(aiCore): add missing @openrouter/ai-sdk-provider dependency
(#13787)

(Skipped internal CI/CD changes)

### Review Checklist

- [ ] Review generated release notes in `electron-builder.yml`
- [ ] Verify version bump in `package.json`
- [ ] CI passes
- [ ] Merge to trigger release build

### Checklist

This checklist is not enforcing, but it's a reminder of items that could
be relevant to every PR.
Approvers are expected to review this list.

- [x] PR: The PR description is expressive enough and will help future
contributors
- [x] Code: [Write code that humans can
understand](https://en.wikiquote.org/wiki/Martin_Fowler#code-for-humans)
and [Keep it simple](https://en.wikipedia.org/wiki/KISS_principle)
- [x] Refactor: You have [left the code cleaner than you found it (Boy
Scout
Rule)](https://learning.oreilly.com/library/view/97-things-every/9780596809515/ch08.html)
- [x] Upgrade: Impact of this change on upgrade flows was considered and
addressed if required
- [x] Documentation: A [user-guide update](https://docs.cherry-ai.com)
was considered and is present (link) or not required. Check this only
when the PR introduces or changes a user-facing feature or behavior.
- [x] Self-review: I have reviewed my own code (e.g., via
[`/gh-pr-review`](/.claude/skills/gh-pr-review/SKILL.md), `gh pr diff`,
or GitHub UI) before requesting review from others

### Release note

```release-note
Cherry Studio 1.8.4 - Bug Fixes and New Features

✨ New Features
- [API] Add knowledge base REST API endpoints
- [SelectionAssistant] Add Linux support for text selection toolbar

🐛 Bug Fixes
- [Files] Fix PDF upload failing with "invalid part type: file" error for OpenAI-compatible providers (Moonshot, DeepSeek, Groq, etc.)
```

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>