chore(desktop): use icon-only logo for app icon

Switch the bundled app icon (.png / .icns / .ico) from the logo-with-text
variant to the icon-only opaque variant — the wordmark was too small to
read at Dock / Finder / Start-menu sizes and looked cluttered. Matches
the macOS Big Sur+ / Windows 11 icon convention where the wordmark
belongs to the product name line, not the tile itself.

Regenerated from resources/logo-variants/不带字不透明.png (1024x1024):
- icon.png — copied verbatim for electron-builder's default fallback.
- icon.icns — assembled via iconutil from the full iconset (16 through
  1024 including @2x variants) for macOS.
- icon.ico — assembled via ImageMagick with auto-resize 16/24/32/48/
  64/128/256 for Windows.

Author: hqhq1025