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>

config.example.json

@@ -35,6 +35,11 @@
       "enabled": false
     }
   ],
+  "origin": {
+    "latitude": 51.4700,
+    "longitude": -0.4600,
+    "name": "Control Tower"
+  },
   "settings": {
     "history_limit": 1000,
     "stale_timeout": 60,