Add Tailscale connection method monitoring

Add connection_method field to NetworkInterfaceData to track whether
Tailscale is using direct P2P, DERP relay, or HTTP proxy connections.
The connection method is displayed as a sub-service under tailscaled
service, following the same pattern as VPN routes and firewall ports.

Query tailscale status --json to determine active connection type and
display as informational sub-service when tailscaled is active.

shared/src/agent_data.rs

@@ -38,6 +38,7 @@ pub struct NetworkInterfaceData {
     pub link_status: Status,
     pub parent_interface: Option<String>,
     pub vlan_id: Option<u16>,
+    pub connection_method: Option<String>, // For Tailscale: "direct", "relay", or "proxy"
 }
 
 /// CPU C-state usage information