feat: external connection handling

[ggfevans]

Parent Epic

Part of #71 (Network Interface Visualization and Connectivity)

Summary

Handle cables that connect to things outside the rack layout (uplinks, wall jacks, ISP handoffs).

Problem Statement

Some cables go to:

• Core switch not in this rack
• Wall jack / patch to another room
• ISP demarcation point
• External equipment (not modeled)

Options

1. External pseudo-device - Add a special "External" device type
2. Labeled endpoint - Cable B-side is just a label, no device
3. Placeholder device - Generic "External Connection" device
4. Data only - Track but don't visualize

Recommended: Labeled Endpoint

interface Cable {
  // A-side (device in rack)
  a_device_id: string;
  a_interface: string;
  
  // B-side (device OR external label)
  b_device_id?: string;      // Optional for external
  b_interface?: string;      // Optional for external
  b_external_label?: string; // "Core Switch", "Wall Jack A1"
}

Acceptance Criteria

• Cable can have external endpoint (no device)
• External endpoint has label
• Visualize external cables (arrow pointing off-rack?)
• Edit external label
• Convert external to internal (when device added to rack)

UI Design

Port details when connected to external:

Connection:
○ External: "Core Switch / uplink1"
[Edit] [Disconnect]

Dependencies

• feat: cable data model and schema #261 (Cable data model)

References

• NetBox handles this via sites/locations