feat(ui): improve dropdown menu contrast with lighter popover background

- Update popover background color from #09090b to #18181b (zinc-900)
- Apply popover background to dropdown menu instead of card color
- Provides better visual separation and elevated appearance

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/app/src/app/agent/[id].tsx

@@ -306,7 +306,7 @@ const styles = StyleSheet.create((theme) => ({
     ...StyleSheet.absoluteFillObject,
   },
   dropdownMenu: {
-    backgroundColor: theme.colors.card,
+    backgroundColor: theme.colors.popover,
     borderRadius: theme.borderRadius.lg,
     padding: theme.spacing[2],
     shadowColor: "#000",

packages/app/src/styles/theme.ts

@@ -129,7 +129,7 @@ const darkSemanticColors = {
   foreground: "#fafafa",
   card: "#09090b",
   cardForeground: "#fafafa",
-  popover: "#09090b",
+  popover: "#18181b",
   popoverForeground: "#fafafa",
   primary: "#fafafa",
   primaryForeground: "#18181b",