v5: standalone Tauri rewrite — Rust backend + TypeScript UI

Complete rewrite from the Python/Tkinter app to a native Tauri 2 desktop
app. Drops the song-detection feature, so it needs no Python and no ML
model — just ffmpeg. Replaces video_concat.py entirely.

- Rust backend (src-tauri): stream-copy concat via ffmpeg with honest
  real-time progress (-progress pipe:1, out_time vs total duration), held
  at 99% through the +faststart finalising pass so the bar never jumps
- ffprobe duration scanning parallelised across threads; cancel kills the
  whole ffmpeg process tree; CREATE_NO_WINDOW hides console flashes
- BiliBili Live filename helper ported to Rust (bililive_rename)
- TypeScript UI, no framework: tiny h() DOM helper, drag-and-drop,
  reorderable/checkable file list with live totals, light/dark theme,
  reusable toast + progress components
- Optional delete-originals-after-merge with a clear warning
- New app icon (three clips merging into one); build.bat assembles a
  portable folder with ffmpeg/ffprobe bundled next to the exe
- Ignore tauri-generated gen/schemas (regenerated on every build)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -0,0 +1,42 @@
+# Video Concat
+
+A small, focused desktop app that **merges split stream recordings into one
+file** using ffmpeg stream-copy (`-c copy`, no re-encoding — fast and lossless).
+Built with Tauri 2 (Rust backend + TypeScript UI).
+
+This is a standalone spin-off of Stream Studio with the song-detection feature
+removed, so it needs **no Python and no ML model** — just ffmpeg.
+
+## Features
+
+- Drag-and-drop / browse to add videos, reorder, select which to merge.
+- Stream-copy concat with honest real-time progress (and a `+faststart`
+  finalising pass so the output seeks instantly).
+- BiliBili-Live filename helper (`[date][uploader][title]` → readable name).
+- Optionally delete the originals after a successful merge.
+
+## Build a standalone (copy-paste) version
+
+Double-click **`build.bat`**. It compiles the app and assembles a portable
+folder with ffmpeg bundled:
+
+```
+dist-portable\VideoConcat\
+  Video Concat.exe   <- double-click this
+  ffmpeg.exe         <- bundled
+  ffprobe.exe        <- bundled
+```
+
+Copy that whole `VideoConcat` folder to any Windows PC and run it — nothing else
+to install.
+
+### Build requirements (on the build machine only)
+
+- [Node.js](https://nodejs.org/)
+- [Rust](https://rustup.rs/) (`winget install Rustlang.Rustup`)
+- Visual Studio 2022 with the **Desktop development with C++** workload
+- ffmpeg (in `C:\ffmpeg\bin` or on `PATH`)
+
+## Develop
+
+Double-click **`dev.bat`** (hot-reload UI + Rust). Needs ffmpeg on `PATH`.