fix(ui): Use 4-column grid for theme picker

8 themes no longer fit in a single row. Changed from flex row
to a 4x2 grid layout so all themes are visible.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/ThemePicker.jsx

@@ -113,9 +113,9 @@ export default function ThemePicker() {
         >
           <div
             style={{
-              display: 'flex',
+              display: 'grid',
-              gap: '16px',
+              gridTemplateColumns: 'repeat(4, 1fr)',
-              justifyContent: 'center',
+              gap: '12px',
             }}
           >
             {themes.map((t) => (
@@ -145,7 +145,7 @@ export default function ThemePicker() {
               >
                 <ThemeSwatch
                   colors={t.swatch}
-                  size={32}
+                  size={28}
                   active={t.id === theme}
                 />
                 <span