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>

src-tauri/icons/android/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+  <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+  <background android:drawable="@color/ic_launcher_background"/>
+</adaptive-icon>