feat: dark theme polish — fix sidebar layout, update button, chat input & theme switching

- Fix sidebar footer layout: separate update button from version row
- Replace custom update-hint with NButton for proper dark mode support
- Darken chat input background in dark theme for better contrast
- Fix system theme listener always applying dark mode

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/composables/useTheme.ts

@@ -29,7 +29,7 @@ applyTheme(resolveDark(mode.value))
 const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)')
 mediaQuery.addEventListener('change', () => {
   if (mode.value === 'system') {
-    applyTheme(true)
+    applyTheme(resolveDark('system'))
   }
 })