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/components/hermes/chat/ChatInput.vue

@@ -356,11 +356,15 @@ function isImage(type: string): boolean {
   border: 1px solid $border-color;
   border-radius: $radius-md;
   padding: 10px 12px;
-  transition: border-color $transition-fast;
+  transition: border-color $transition-fast, background-color $transition-fast;
 
   &:focus-within {
     border-color: $accent-primary;
   }
+
+  .dark & {
+    background-color: #333333;
+  }
 }
 
 .input-textarea {