feat(media): extract audio track to local file (#39)

[cuic19053-hue]

Summary

End-to-end "extract audio" path so users can save a video's soundtrack as a standalone audio file from the media panel. Closes #39.

Changes

Backend (Rust)

• crates/opentake-media/src/lib.rs — MediaEngine::extract_audio + extract_audio_file helper drive ffmpeg via the existing ffmpeg_path() CLI wrapper. Invokes -y -i <in> -vn plus codec args picked by output extension:
  • .m4a / .aac → AAC 192k
  • .mp3 → libmp3lame 192k
  • .wav → pcm_s16le
• src-tauri/src/media.rs — new extract_audio Tauri command resolves a media id to its MediaSource::External absolute path, validates the file exists, then delegates to the engine. Returns the output path.
• src-tauri/src/lib.rs — register media::extract_audio in generate_handler!.

Frontend (React/TS)

• web/src/lib/api.ts — extractAudio(mediaId, outPath) wrapper; rejects outside Tauri (no ffmpeg available).
• web/src/components/media/MediaPanel.tsx — MediaCard gains a star-shaped "Extract Audio" button on the top-left, shown only when hovering a video that carries an audio track. Click opens a native save dialog (m4a/mp3/wav filters), invokes extract_audio, and surfaces a transient success/failure feedback message. stopPropagation+preventDefault keep the click from selecting the card.
• web/src/i18n/dict.ts — 6 new keys (zh-CN + en) for the button title, hint, success, failure, and no-audio messages.

Verification

• pnpm tsc --noEmit ✅
• pnpm build ✅ (vite production build, 1639 modules)
• Rust changes follow existing patterns (mirror MediaEngine method + Tauri command); CI will verify cargo build.

User flow

1. Hover a video asset in the media panel that has an audio track.
2. Click the ★ star icon (top-left of the card).
3. Native save dialog opens with .m4a / .mp3 / .wav filters.
4. ffmpeg muxes the audio track to the chosen path.
5. A transient toast confirms the saved path (or shows the error).

Notes

• Only MediaSource::External assets are supported (project-relative assets return an error). This matches the existing import_folder / import_media path which also requires absolute external files.
• Output codec is selected by file extension so the user can pick the format they need without an extra UI control.

[appergb]

@cuic19053-hue 自动审核结论:请修改(REQUEST_CHANGES)。Rust 抽音轨逻辑结构正确,但无法直接合并:

阻塞项:

1. 冲突,需 rebase:diff 基于 PR#102 之前的旧版。src-tauri/src/lib.rs 上下文缺 media::get_waveform 行 → patch 应用失败,extract_audio 命令注册不上;MediaPanel.tsx 也是旧版上下文。请 rebase 到最新 main。
2. UI 叠层冲突(严重):main 的 MediaCard 已有收藏星标固定在 position:absolute left:4 top:4;本 PR 在同坐标再放一个 Star 做"提取音频",两按钮叠加、点击错乱。上游抽音频走右键菜单 Save as Media(EditorViewModel+SaveAsMedia.swift),不用星标。请换图标/位置或改走右键菜单(与 #93/#101 协调)。
3. 缺 issue #39 验收测试:提取后文件存在 / 时长匹配 / 无视频流 三项测试缺失,请补集成测试。
4. out_path 无后端路径边界校验(任意 ../ 可写)。

注:本 PR 触碰 media.rs/MediaPanel,属 #91 重写范围,建议与 #91/#101 协调以免白做。请 rebase + 重设计 UI + 补测试后重提。