Create asset module and add origin configuration

Asset Module:
- Created internal/assets package for clean embedded static file management
- Embedded static files from single source location (static/ directory)
- Eliminated need for duplicate static files or symlinks
- Assets accessible via assets.Static from any package

Origin Configuration:
- Added origin field to configuration with latitude, longitude, and name
- Automatically calculates origin as average of source positions if not specified
- Displays origin information on startup
- Updated config.example.json to show origin usage

Architecture:
- Moved main.go back to cmd/skyview/ structure
- Updated Makefile to build from cmd/skyview
- Clean separation of concerns with reusable asset module

This provides a robust foundation for asset management and proper origin
handling for distance calculations and map centering.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/assets/static/aircraft-icon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="#00a8ff" stroke="#ffffff" stroke-width="1">
+  <path d="M12 2l-2 16 2-2 2 2-2-16z"/>
+  <path d="M4 10l8-2-1 2-7 0z"/>
+  <path d="M20 10l-8-2 1 2 7 0z"/>
+</svg>