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/QuickSettingsPanelHeader.tsx

@@ -5,9 +5,9 @@ export default function QuickSettingsPanelHeader() {
   const { t } = useTranslation('settings');
 
   return (
-    <div className="border-b border-gray-200 bg-gray-50 p-4 dark:border-gray-700 dark:bg-gray-900">
-      <h3 className="flex items-center gap-2 text-lg font-semibold text-gray-900 dark:text-white">
-        <Settings2 className="h-5 w-5 text-gray-600 dark:text-gray-400" />
+    <div className="border-b border-border bg-muted/40 p-4">
+      <h3 className="flex items-center gap-2 text-lg font-semibold text-foreground">
+        <Settings2 className="h-5 w-5 text-muted-foreground" />
         {t('quickSettings.title')}
       </h3>
     </div>