feat: port position validation for half-depth devices

[ggfevans]

Parent Epic

Part of #71 (Network Interface Visualization and Connectivity)

Summary

Ensure interface port positions are valid for the device's depth configuration.

Problem Statement

• Half-depth devices (is_full_depth: false) only occupy front OR rear face
• Interfaces with position: 'rear' on a front-only device are invalid
• Need validation and/or auto-correction

Acceptance Criteria

• Add validation in Zod schema (custom refinement)
• Warn user if port position conflicts with device depth
• Consider: auto-assign position based on device face placement
• Document behavior in SPEC.md

Validation Rule

// If device is half-depth and placed on front face:
// All interfaces should have position: 'front' (or undefined, defaulting to front)

Dependencies

• feat: expand InterfaceTemplate type and Zod schema #247 (InterfaceTemplate schema)

References

• Half-depth logic: src/lib/components/Rack.svelte line 176-183