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>

build.bat

@@ -0,0 +1,52 @@
+@echo off
+REM ============================================================
+REM Video Concat - build a STANDALONE, portable Windows app.
+REM
+REM   Output:  dist-portable\VideoConcat\
+REM   Usage:   copy that whole folder to any Windows PC and
+REM            double-click "Video Concat.exe". ffmpeg is bundled.
+REM
+REM   Requires on THIS build machine: Node, Rust (cargo),
+REM   Visual Studio 2022 C++ build tools, and ffmpeg.
+REM ============================================================
+setlocal EnableDelayedExpansion
+set "PATH=%USERPROFILE%\.cargo\bin;%PATH%"
+cd /d "%~dp0"
+
+REM --- 1. Locate ffmpeg.exe + ffprobe.exe --------------------
+set "FFMPEG="
+set "FFPROBE="
+if exist "C:\ffmpeg\bin\ffmpeg.exe"  set "FFMPEG=C:\ffmpeg\bin\ffmpeg.exe"
+if exist "C:\ffmpeg\bin\ffprobe.exe" set "FFPROBE=C:\ffmpeg\bin\ffprobe.exe"
+if not defined FFMPEG  for /f "delims=" %%i in ('where ffmpeg 2^>nul')  do if not defined FFMPEG  set "FFMPEG=%%i"
+if not defined FFPROBE for /f "delims=" %%i in ('where ffprobe 2^>nul') do if not defined FFPROBE set "FFPROBE=%%i"
+
+if not defined FFMPEG  ( echo [ERROR] ffmpeg.exe not found. Install ffmpeg or place it in C:\ffmpeg\bin & pause & exit /b 1 )
+if not defined FFPROBE ( echo [ERROR] ffprobe.exe not found. Install ffmpeg or place it in C:\ffmpeg\bin & pause & exit /b 1 )
+echo Using ffmpeg:  !FFMPEG!
+echo Using ffprobe: !FFPROBE!
+
+REM --- 2. Build the release app ------------------------------
+if not exist "node_modules" call npm install
+echo Building release (this takes a few minutes the first time)...
+call npm run tauri build
+if errorlevel 1 ( echo [ERROR] Build failed. & pause & exit /b 1 )
+
+REM --- 3. Assemble the portable folder ----------------------
+set "OUT=dist-portable\VideoConcat"
+if exist "dist-portable" rmdir /s /q "dist-portable"
+mkdir "%OUT%"
+copy /y "src-tauri\target\release\video-concat.exe" "%OUT%\Video Concat.exe" >nul
+copy /y "!FFMPEG!"  "%OUT%\ffmpeg.exe"  >nul
+copy /y "!FFPROBE!" "%OUT%\ffprobe.exe" >nul
+
+echo.
+echo ============================================================
+echo  DONE. Your standalone app is here:
+echo    %CD%\%OUT%
+echo.
+echo  Copy that whole "VideoConcat" folder to any Windows PC and
+echo  double-click "Video Concat.exe". ffmpeg is included, so
+echo  there is nothing else to install.
+echo ============================================================
+pause