docs: Write display modes and visualization guide

[ggfevans]

Summary

Document display modes, airflow visualization, and visual controls in Rackula.

Content Outline

Display Modes

• Label mode: Device names as text
• Image mode: Device images only
• Image-label mode: Images with name overlay
• Toggle with 'I' key or toolbar button

Airflow Visualization

• Edge stripe indicators for airflow direction
• Arrow overlays showing flow patterns
• Conflict detection (orange border highlighting)
• Toggle with 'A' key
• Support in exports
• 7 airflow types: passive, front-to-rear, rear-to-front, left-to-right, right-to-left, side-to-rear, mixed

Front/Rear Views

• Front and rear view toggle
• Device face filtering by view
• Dual view for side-by-side comparison
• View labels in exports

Device Visualization

• Category icons for identification
• Device labels with auto-sizing text
• Placeholder icons for devices without images
• Image shimmer loading state

Acceptance Criteria

• All display modes documented with screenshots
• Airflow visualization explained
• Front/rear view controls documented
• Keyboard shortcuts mentioned

Test Requirements

• Build completes without errors
• Screenshots are current