Change tree symbols to blue color across all panels

- Add Typography::tree() style using blue Theme::highlight() color
- Update system, backup, and services widgets to use consistent blue tree styling
- Centralizes tree color management in theme module for easy maintenance

dashboard/src/ui/theme.rs

@@ -324,4 +324,11 @@ impl Typography {
             .bg(Theme::background())
             .add_modifier(Modifier::BOLD)
     }
+
+    /// Tree symbols style (blue color)
+    pub fn tree() -> Style {
+        Style::default()
+            .fg(Theme::highlight())
+            .bg(Theme::background())
+    }
 }