refactor: add shared StoplightColors for consistent status colors

- Add StoplightColors class with green/orange/red pastel colors
- Update dashboard gauges to use StoplightColors.forPercent()
- Update air quality levels to use shared stoplight colors
- Provides consistent color scheme for all threshold-based indicators

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/shared/theme/stoplight_colors.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/material.dart';
+
+/// Pastel stoplight colors for consistent status indication across the app.
+///
+/// Use these colors for any green/yellow/red status flows:
+/// - System stats (CPU, RAM, disk usage)
+/// - Air quality levels
+/// - Container health status
+/// - Any other threshold-based indicators
+abstract final class StoplightColors {
+  /// Good/healthy/low usage (0-50%)
+  static const green = Color(0xFF81C784);
+
+  /// Warning/moderate/medium usage (51-75%)
+  static const orange = Color(0xFFFFB74D);
+
+  /// Critical/unhealthy/high usage (76-100%)
+  static const red = Color(0xFFE57373);
+
+  /// Returns appropriate color based on percentage (0-100).
+  ///
+  /// - ≤50%: green
+  /// - 51-75%: orange
+  /// - >75%: red
+  static Color forPercent(double percent) {
+    if (percent <= 50) return green;
+    if (percent <= 75) return orange;
+    return red;
+  }
+
+  /// Returns appropriate color based on value (0.0-1.0).
+  ///
+  /// - ≤0.5: green
+  /// - 0.51-0.75: orange
+  /// - >0.75: red
+  static Color forValue(double value) => forPercent(value * 100);
+}