Add official theming support (accent color, font, spacing config)

QwenPaw Desktop has no way to change its look. The UI is locked to one orange accent (#f07e26), a system font, and fixed spacing. There is no config key, no settings page, and no CSS injection point.

I've been customizing by editing index.html inside the .app bundle — it works, but every app update wipes it out.

Request: add a "theme" section to config.json that maps to the CSS custom properties the app already uses (--colorPrimary, --colorBgBase, --font-family, --border-radius, etc). Something like:

{
"theme": {
"accent": "#0b57d0",
"accent_hover": "#1967d2",
"accent_bg": "#e8f0fe",
"font_family": "Google Sans Text, Roboto, sans-serif",
"font_family_mono": "SF Mono, Menlo, monospace",
"radius": "12px",
"dark": {
"accent": "#8ab4f8",
"accent_bg": "rgba(138,180,248,0.15)",
"surface": "#1a1a1a"
}
}
}

The React app already consumes these variables everywhere — we just need to let users set them. A small "Appearance" panel in settings (color picker + font select + radius slider) would round it out.

Defaults stay unchanged if no theme key is present. Happy to write the PR.

Version: [paste qwenpaw --version output]
Platform: macOS arm64