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

Start by tracing config.json into the React app and locating the existing CSS custom properties; the issue names index.html inside the .app bundle as the current workaround. Confirm how settings are assembled before defining the theme controls. Done means configurable theme values, an Appearance panel for color, font, and radius settings, dark overrides, and unchanged defaults when no theme key is present.

Scritto dal modello di indicizzazione a partire dal testo della issue.