Improve Beast decoder with proper data formatting and altitude decoding

Major improvements to Beast message decoding and data presentation:

Speed & Track Formatting:
- Convert ground speed from float to integer (knots)
- Convert track angle from float to integer (0-359 degrees)
- Add proper rounding for velocity calculations
- Fix surface movement speed calculations

Altitude Decoding Enhancement:
- Implement proper Q-bit handling in altitude decoding
- Add altitude validation (range: -1000 to 60000 feet)
- Fix standard altitude encoding with 25-foot increments
- Add legacy Gray code support for older transponders
- Remove duplicate altitude values caused by incorrect decoding

ICAO Address Display:
- Fix JavaScript hex conversion that caused display corruption
- Remove duplicate toString(16) calls on already-formatted hex strings
- Proper ICAO display format (6-character hex like "4ACA0C")

Data Type Consistency:
- Update Aircraft struct to use integers for GroundSpeed and Track
- Update SpeedPoint struct for consistent integer speed/track storage
- Maintain float64 precision internally while displaying clean integers

Signal Strength:
- Confirm signal strength extraction is working properly
- Signal levels properly flow from Beast parser through merger to frontend
- Display in dBFS format (e.g., -1.57 dBFS)

Results:
- Clean integer speed values (198 kt instead of 238.03361107205006 kt)
- Proper track angles (41° instead of 37.83276127148023°)
- Realistic varying altitudes (1750-1825 ft instead of repeated 24450 ft)
- Correct ICAO formatting (4ACA0C instead of corrupted 103A)
- Working signal strength display (-0.98 to -2.16 dBFS)

The Beast decoder now produces accurate, properly formatted aircraft data
that displays cleanly in the web interface without data corruption.

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

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

internal/merger/merger.go

@@ -113,8 +113,8 @@ type AltitudePoint struct {
 // Used for aircraft performance analysis and track prediction.
 type SpeedPoint struct {
 	Time        time.Time `json:"time"`         // Timestamp when speed was received
-	GroundSpeed float64   `json:"ground_speed"` // Ground speed in knots
-	Track       float64   `json:"track"`        // Track angle in degrees
+	GroundSpeed int       `json:"ground_speed"` // Ground speed in knots (integer)
+	Track       int       `json:"track"`        // Track angle in degrees (0-359)
 }
 
 // Merger handles merging aircraft data from multiple sources with intelligent conflict resolution.