style(theme): lighten global text-selection highlight

Native ::selection was near-invisible on dark surfaces, hiding selected
text inside inputs. Add a global ::selection using the hover-selection
treatment (gray.200 light / whiteAlpha.300 dark) and leave the text
color untouched so contrast holds everywhere.

src/styles/theme.tsx

@@ -349,6 +349,11 @@ const config = defineConfig({
       bg: { base: 'gray.200', _dark: 'gray.800' },
       fontSize: '1.0625rem',
     },
+    // Native text-selection was near-invisible on dark surfaces.
+    // Match the hover-selection treatment (gray.200 / whiteAlpha.300).
+    '::selection': {
+      bg: { base: 'gray.200', _dark: 'whiteAlpha.300' },
+    },
   },
 })