Windows support: cross-built NSIS installer

- toFileURL and path-basename handling made Windows-safe (backslashes,
  drive letters) so media/model loading works there
- Blender discovery scans Program Files\Blender Foundation; ffmpeg
  candidates include common Windows paths
- build config: win/nsis target (x64, custom install dir), build/icon.ico
  generated from the 1024 png, dist:win script
Cross-compiled from macOS: dist/DJ Visualizer Setup 1.6.0.exe (95MB) with
scripts unpacked and models/MediaPipe bundled. Untested on real Windows.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -10,7 +10,8 @@
   "license": "MIT",
   "scripts": {
     "start": "electron .",
-    "dist": "electron-builder --mac"
+    "dist": "electron-builder --mac",
+    "dist:win": "electron-builder --win"
   },
   "build": {
     "appId": "com.djvisualizer.app",
@@ -22,6 +23,21 @@
       "icon": "build/icon.icns",
       "identity": null
     },
+    "win": {
+      "target": [
+        {
+          "target": "nsis",
+          "arch": [
+            "x64"
+          ]
+        }
+      ],
+      "icon": "build/icon.ico"
+    },
+    "nsis": {
+      "oneClick": false,
+      "allowToChangeInstallationDirectory": true
+    },
     "files": [
       "main.js",
       "preload.js",