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/chat/view/subcomponents/MessageComponent.tsx

@@ -377,15 +377,15 @@ const MessageComponent = memo(({ message, prevMessage, createDiff, onFileOpen, a
 
                       return (
                         <div className="my-2">
-                          <div className="mb-2 flex items-center gap-2 text-sm text-gray-600 dark:text-gray-400">
+                          <div className="mb-2 flex items-center gap-2 text-sm text-muted-foreground">
                             <svg className="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                               <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M7 21h10a2 2 0 002-2V9.414a1 1 0 00-.293-.707l-5.414-5.414A1 1 0 0012.586 3H7a2 2 0 00-2 2v14a2 2 0 002 2z" />
                             </svg>
                             <span className="font-medium">{t('json.response')}</span>
                           </div>
-                          <div className="overflow-hidden rounded-lg border border-gray-600/30 bg-gray-800 dark:border-gray-700 dark:bg-gray-900">
+                          <div className="overflow-hidden rounded-lg border border-border bg-muted">
                             <pre className="overflow-x-auto p-4">
-                              <code className="block whitespace-pre font-mono text-sm text-gray-100 dark:text-gray-200">
+                              <code className="block whitespace-pre font-mono text-sm text-foreground">
                                 {formatted}
                               </code>
                             </pre>