style(ui): rework light/dark theme to make it visually consistent

Rework the color system around warm neutrals and route hardcoded
surfaces through theme tokens for consistency.

- Theme tokens (index.css, ThemeContext): warm cream light mode and
  neutral charcoal dark mode, replacing the pure-white/blue-tinted
  palette; update PWA theme-color meta
- Code blocks: soft grey background in light mode via
  oneLight/oneDark, and drop the Tailwind Typography <pre> shell that
  framed the highlighter in a dark box
- Dropdowns/panels: convert CommandMenu, Quick Settings, and the JSON
  response block from hardcoded gray/slate to popover/muted/border
  tokens
- Git panel: Publish button purple -> primary blue
- Composer: drop top padding so the input sits flush with the thread

src/components/quick-settings-panel/view/QuickSettingsToggleRow.tsx

@@ -17,8 +17,8 @@ function QuickSettingsToggleRow({
 }: QuickSettingsToggleRowProps) {
   return (
     <label className={TOGGLE_ROW_CLASS}>
-      <span className="flex items-center gap-2 text-sm text-gray-900 dark:text-white">
-        <Icon className="h-4 w-4 text-gray-600 dark:text-gray-400" />
+      <span className="flex items-center gap-2 text-sm text-foreground">
+        <Icon className="h-4 w-4 text-muted-foreground" />
         {label}
       </span>
       <input