feat(sun-position): add proportional arc visualization with 10-min updates

- Arc angle now proportional to day/night duration (day = daylight/24 × 360°)
- Horizon points represent sunrise/sunset times
- Day arc with sun icon and yellow/orange gradient
- Night arc with moon icon and blue/indigo gradient
- Position updates every 10 minutes aligned to clock (0/10/20/30/40/50)
- Default to 07:00-17:00 when API data unavailable (asymmetric for visual effect)
- All environment cards maintain consistent height in "no data" state

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

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

lib/shared/widgets/forecast_widget.dart

@@ -27,13 +27,50 @@ class ForecastWidget extends StatelessWidget {
       return Card(
         child: Padding(
           padding: const EdgeInsets.all(16),
-          child: Center(
-            child: Text(
-              'No forecast data available',
-              style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              // Header
+              Row(
+                children: [
+                  Icon(
+                    Icons.calendar_today,
+                    size: 20,
                     color: colorScheme.onSurfaceVariant,
                   ),
-            ),
+                  const SizedBox(width: 8),
+                  Text(
+                    'Forecast',
+                    style: Theme.of(context).textTheme.titleSmall?.copyWith(
+                          color: colorScheme.onSurfaceVariant,
+                        ),
+                  ),
+                ],
+              ),
+              // Match height of populated card content area
+              const SizedBox(height: 120),
+              Center(
+                child: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: [
+                    Icon(
+                      Icons.event_busy_outlined,
+                      size: 32,
+                      color: colorScheme.outline,
+                    ),
+                    const SizedBox(height: 8),
+                    Text(
+                      'No data available',
+                      style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+                            color: colorScheme.outline,
+                          ),
+                    ),
+                  ],
+                ),
+              ),
+              const SizedBox(height: 40),
+            ],
           ),
         ),
       );