docs(website): 新增 FFmpeg 组件安装说明文档

补充浏览器扩展使用文档,说明 DASH/HLS 流分离音视频轨道时需安装 FFmpeg 才能合并输出,中英双语同步。

Author: zerx-lab

website/src/content/docs/en/browser-extension/usage.md

@@ -69,6 +69,10 @@ The extension's toolbar icon shows a numeric badge with the resource count for t
 
 <!-- TODO(screenshot): floating ball docked to the page edge next to an open resource panel with tabs and a batch selection -->
 
+### Install the FFmpeg component for merged output
+
+Many of the resources you'll sniff — streaming manifests especially — don't arrive as one finished file. DASH streams (`.mpd`) routinely split audio and video into separate tracks, so downloading one produces two files (for example `video.mp4` and `video.audio.m4a`) unless FluxDown can merge them. That merge is done with **FFmpeg** (a fast stream copy, no re-encoding), which isn't bundled with the app — it's an optional component you install on demand from **Settings → Components** inside the desktop app. With it installed, DASH audio/video tracks are merged into a single playable file automatically; without it, the tracks are kept separate for you to merge yourself. Installing FFmpeg also unlocks plugins that declare the `ffmpeg` capability (see the [plugin API reference](/docs/en/plugins/api-reference/)), and installs `ffprobe` alongside it. HLS (`.m3u8`) downloads don't need FFmpeg — FluxDown reassembles and remuxes those to MP4 on its own.
+
 ## Track today's stats
 
 The popup's **Today's Stats** section shows two running counters for the current day: **Intercepted** (downloads successfully handed off to FluxDown) and **Failed** (hand-offs that didn't go through, typically because the desktop app was unreachable at the time). Both reset automatically at the start of a new day, or immediately if you click **Reset Stats** in the popup's footer.

website/src/content/docs/zh/browser-extension/usage.md

@@ -3,7 +3,7 @@ title: 使用扩展
 description: 下载拦截的工作方式、三种拦截模式、资源嗅探与弹窗常用控制项。
 section: browser-extension
 order: 2
-sourceHash: "ac627a078874"
+sourceHash: "5c5d3da6c527"
 ---
 
 ## 下载拦截的工作方式
@@ -70,6 +70,10 @@ FluxDown 会持续扫描每个页面中的可下载资源——视频、音频
 
 <!-- TODO(screenshot): 悬浮球停靠在页面边缘,旁边展开资源面板,可见标签分类与批量选择 -->
 
+### 安装 FFmpeg 组件以合并输出
+
+你嗅探到的资源里,有不少——尤其是流媒体清单——并不会以单个成品文件的形式到手。DASH 流(`.mpd`)通常会把音频和视频拆成两条独立轨道,因此下载后会得到两个文件(例如 `video.mp4` 和 `video.audio.m4a`),除非 FluxDown 能把它们合并。这一合并由 **FFmpeg** 完成(纯流复制,不重新编码,速度很快),它不随应用打包分发——而是一个可选组件,需要你在桌面客户端的**设置 → 组件**页按需安装。装好后,DASH 的音视频轨会自动合并成一个可直接播放的文件;不装则两条轨道保留为独立文件,需要你自己合并。安装 FFmpeg 还会一并解锁声明了 `ffmpeg` 能力的插件(见[插件 API 参考](/docs/zh/plugins/api-reference/)),并同时安装 `ffprobe`。HLS(`.m3u8`)下载不需要 FFmpeg——FluxDown 会自行把它们重组并 remux 成 MP4。
+
 ## 查看今日统计
 
 弹窗的**今日统计**区域显示当天的两个计数:**已接管**(成功转交给 FluxDown 的下载数)与**失败**(未能成功转交的次数,通常是因为当时连不上桌面客户端)。两个计数都会在每天开始时自动清零,也可以点击弹窗底部的**重置统计**立即清零。