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/widgets/air_quality_widget.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:tatlock_ui/shared/theme/stoplight_colors.dart';
 
 /// Air Quality Index widget displaying current AQI.
 ///
@@ -203,28 +204,28 @@ enum AqiLevel {
   good(
     label: 'Good',
     description: 'Air quality is satisfactory',
-    color: Colors.green,
+    color: StoplightColors.green,
     minIndex: 0,
     maxIndex: 50,
   ),
   moderate(
     label: 'Moderate',
     description: 'Acceptable for most people',
-    color: Colors.amber,
+    color: StoplightColors.orange,
     minIndex: 51,
     maxIndex: 100,
   ),
   unhealthySensitive(
     label: 'Unhealthy for Sensitive',
     description: 'May affect sensitive groups',
-    color: Colors.orange,
+    color: StoplightColors.orange,
     minIndex: 101,
     maxIndex: 150,
   ),
   unhealthy(
     label: 'Unhealthy',
     description: 'Health effects for everyone',
-    color: Colors.red,
+    color: StoplightColors.red,
     minIndex: 151,
     maxIndex: 200,
   ),