Fix label contrast against background in dark mode

[Copilot]

Labels in the string art configuration UI become invisible when system dark mode is active — containers have hardcoded background: white but text color is inherited from :root which flips to white in dark mode.

Changes

• Explicit text color on light containers: Added color: #333 to .generation-controls and .upload-section so labels are always readable regardless of inherited color
• Dark mode overrides in App.css: Added @media (prefers-color-scheme: dark) block switching containers to dark backgrounds with light text
• Upload component dark mode: Added dark mode styles for .image-uploader and .upload-text
• Canvas container: Added explicit color: #333 to .string-art-canvas-container

/* Before: labels inherit white color from :root in dark mode, invisible on white bg */
.generation-controls {
  background: white;
}

/* After: explicit contrast + dark mode override */
.generation-controls {
  background: white;
  color: #333;
}

@media (prefers-color-scheme: dark) {
  .generation-controls {
    background: #1f1f1f;
    color: rgba(255, 255, 255, 0.87);
  }
}