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>

.gitignore

@@ -1,4 +1,42 @@
-video_concat_v2.1.py
-video_concat_v3.py
-video_concat_v4.py
-video_concat_v4.1.py
+# deps & caches
+node_modules/
+__pycache__/
+*.pyc
+.venv/
+venv/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+# build outputs
+target/
+dist/
+dist-portable/
+build/
+# tauri auto-generated (regenerated on every build)
+/src-tauri/gen/schemas/
+# logs & databases
+*.log
+*.db
+*.sqlite
+*.sqlite3
+# secrets / credentials
+.env
+*.env
+*cookie*
+*credential*
+credentials.json
+token.json
+*token.json
+.cookies.json
+google_cred/
+*.pem
+*.key
+# model weights / large artifacts
+*.onnx
+*.pt
+*.pth
+*.ckpt
+*.safetensors
+# os cruft
+.DS_Store
+Thumbs.db