build: add Windows portable EXE packaging scripts

- scripts/build-icon.js — generates icon.png/ico/tray-icon.png
  from pure Node.js (no dependencies, zlib + PNG/ICO encoder)
- scripts/fix-wincodeSign.js — pre-populates electron-builder
  winCodeSign cache by extracting the .7z while ignoring macOS
  symlinks (which fail on Windows without SeCreateSymbolicLink)
- package.json: add fix-wincodeSign script, cross-env, electron-builder
  downgraded to v24 (stable portable target), CSC signing disabled

Build: npm run build:win → dist-electron/SRT-Streamer-Portable-1.0.0.exe

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

.gitignore

@@ -1,6 +1,9 @@
 node_modules/
 dist/
 dist-electron/
+assets/icon.png
+assets/icon.ico
+assets/tray-icon.png
 ffmpeg-bin/ffmpeg
 ffmpeg-bin/ffmpeg.exe
 ffmpeg-bin/ffprobe