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>,