feat: add Windows setup script and launcher for running from source

- setup-windows.ps1: downloads libmpv DLL and ffmpeg, installs pip deps
- 8cut.bat: double-click launcher
- main.py: add_dll_directory for libmpv next to script (not just frozen)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

setup-windows.ps1

@@ -0,0 +1,59 @@
+# 8-cut Windows setup script
+# Run once: powershell -ExecutionPolicy Bypass -File setup-windows.ps1
+#
+# Prerequisites: Python 3.10+ must be installed and on PATH
+#   https://www.python.org/downloads/
+
+$ErrorActionPreference = "Stop"
+$root = Split-Path -Parent $MyInvocation.MyCommand.Path
+
+Write-Host "=== 8-cut Windows Setup ===" -ForegroundColor Cyan
+
+# ── Python deps ────────────────────────────────────────────
+Write-Host "`nInstalling Python dependencies..."
+pip install PyQt6 python-mpv
+
+# ── libmpv ─────────────────────────────────────────────────
+$mpvDll = Join-Path $root "libmpv-2.dll"
+if (Test-Path $mpvDll) {
+    Write-Host "`nlibmpv-2.dll already present, skipping." -ForegroundColor Green
+} else {
+    Write-Host "`nDownloading libmpv..."
+    $release = Invoke-RestMethod "https://api.github.com/repos/shinchiro/mpv-winbuild-cmake/releases/latest"
+    $asset = $release.assets | Where-Object { $_.name -like "mpv-dev-x86_64-v3-*" } | Select-Object -First 1
+    $tmpFile = Join-Path $root "mpv-dev.7z"
+    Invoke-WebRequest $asset.browser_download_url -OutFile $tmpFile
+    7z x $tmpFile -o"$root\mpv-dev" -y | Out-Null
+    Copy-Item "$root\mpv-dev\libmpv-2.dll" $root
+    Remove-Item $tmpFile -Force
+    Remove-Item "$root\mpv-dev" -Recurse -Force
+    Write-Host "libmpv-2.dll downloaded." -ForegroundColor Green
+}
+
+# ── ffmpeg ─────────────────────────────────────────────────
+$ffmpeg = Join-Path $root "ffmpeg.exe"
+if (Test-Path $ffmpeg) {
+    Write-Host "`nffmpeg.exe already present, skipping." -ForegroundColor Green
+} else {
+    # Check if ffmpeg is on PATH
+    $onPath = Get-Command ffmpeg -ErrorAction SilentlyContinue
+    if ($onPath) {
+        Write-Host "`nffmpeg found on PATH: $($onPath.Source)" -ForegroundColor Green
+    } else {
+        Write-Host "`nDownloading ffmpeg..."
+        $ffUrl = "https://github.com/BtbN/FFmpeg-Builds/releases/download/latest/ffmpeg-master-latest-win64-gpl.zip"
+        $tmpZip = Join-Path $root "ffmpeg.zip"
+        Invoke-WebRequest $ffUrl -OutFile $tmpZip
+        Expand-Archive $tmpZip -DestinationPath "$root\ffmpeg-tmp" -Force
+        $bin = Get-ChildItem -Path "$root\ffmpeg-tmp" -Recurse -Filter ffmpeg.exe | Select-Object -First 1
+        Copy-Item "$($bin.DirectoryName)\ffmpeg.exe" $root
+        Copy-Item "$($bin.DirectoryName)\ffprobe.exe" $root
+        Remove-Item $tmpZip -Force
+        Remove-Item "$root\ffmpeg-tmp" -Recurse -Force
+        Write-Host "ffmpeg.exe downloaded." -ForegroundColor Green
+    }
+}
+
+Write-Host "`n=== Setup complete ===" -ForegroundColor Cyan
+Write-Host "Run 8-cut with: python main.py"
+Write-Host "Or double-click: 8cut.bat"