Range Outline with Altitude-Based Gradient Visualization
This PR adds a new Range Outline feature that visualizes the maximum detection range of your ADS-B receiver across all bearings (0-359°), with
altitude-based color gradients showing your coverage profile.

What it does

• Tracks maximum range at each degree bearing from the receiver
• Displays altitude-colored outline on the map using the existing altitude color scale
• Creates smooth gradients between adjacent bearings for visual continuity
• Persists data across restarts with configurable retention period
• Adapts dynamically as aircraft are tracked, expanding outward from the receiver location

Visual Example
The outline starts as a small circle at your receiver location and gradually expands as aircraft are detected. Each segment is colored according to the
altitude of the aircraft at maximum range for that bearing:

• Purple/magenta for high-altitude aircraft (35,000+ ft)
• Blue/cyan for medium altitude (15,000-30,000 ft)
• Green/yellow for lower altitude (below 15,000 ft)

Configuration
New command-line option:
--range-outline-retention <hours> # Default: 24 hours

Example:
./dump1090 --lat 42.67 --lon -113.87 --range-outline-retention 48

UI Controls

• Toggle via checkbox in the web interface settings panel
• State persists across browser sessions
• Works alongside existing site circles and range rings

Technical Details
Backend (C):

• Tracks range, timestamp, and altitude for each of 360 bearings
• Prefers barometric altitude, falls back to geometric
• Binary persistence format with automatic save every 60 seconds
• JSON API endpoint: data/range_outline.json

Frontend (JavaScript):

• 360 individual line segments with altitude-based coloring
• Uses existing getAltitudeColor() function for consistency
• Smooth color gradients between adjacent bearings
• Always displays complete circle (small radius for bearings without data)

Files Modified

• dump1090.h - Data structures
• dump1090.c - Persistence and configuration
• track.c - Range/altitude tracking
• net_io.c - JSON generation
• public_html/script.js - Visualization
• public_html/index.html - UI controls

Documentation
Complete documentation in RANGE_OUTLINE.md including:

• Technical implementation details
• Configuration options
• Troubleshooting guide
• Design decisions