Add disk quota display to services widget

Implement disk quota/total display in services widget showing usage/quota
format. When services don't have specific disk quotas configured, use
system total disk capacity as the quota value.

Changes:
- Add disk_quota_gb field to ServiceData struct in agent
- Add disk_quota_gb field to ServiceInfo struct in dashboard
- Update format_disk_value to show usage/quota format
- Use system disk total capacity as default quota for services
- Rename DiskUsage.total_gb to total_capacity_gb for clarity

Services will now display disk usage as "5.2/500.0 GB" format where
500.0 GB is either the service's specific quota or system total capacity.

dashboard/src/data/metrics.rs

@@ -117,6 +117,8 @@ pub struct ServiceInfo {
     #[serde(default)]
     pub disk_used_gb: f32,
     #[serde(default)]
+    pub disk_quota_gb: f32,
+    #[serde(default)]
     pub description: Option<Vec<String>>,
     #[serde(default)]
     pub sub_service: Option<String>,

dashboard/src/ui/services.rs

@@ -126,7 +126,7 @@ fn render_metrics(
                     svc.name.clone(),
                     format_memory_value(svc.memory_used_mb, svc.memory_quota_mb),
                     format_cpu_value(svc.cpu_percent),
-                    format_disk_value(svc.disk_used_gb),
+                    format_disk_value(svc.disk_used_gb, svc.disk_quota_gb),
                 ],
             );
         }
@@ -158,14 +158,19 @@ fn format_cpu_value(cpu_percent: f32) -> String {
     }
 }
 
-fn format_disk_value(used: f32) -> String {
-    if used >= 1.0 {
+fn format_disk_value(used: f32, quota: f32) -> String {
+    if quota > 0.05 {
+        // Show usage/quota format when quota is set
+        format!("{:.1}/{:.1} GB", used, quota)
+    } else if used >= 1.0 {
         format!("{:.1} GB", used)
     } else if used >= 0.001 {
         // 1 MB or more  
         format!("{:.0} MB", used * 1000.0)
-    } else {
+    } else if used > 0.0 {
         "<1 MB".to_string()
+    } else {
+        "—".to_string() // Em dash for no disk usage
     }
 }