Wegpunkt-Pfeile mit Richtung und Info-Fenster; GPX-Schreib-Performance fix

- TrackDetailActivity: Zeigt zoom-adaptive Richtungspfeile (Bearing) als Marker auf der Karte; Klick öffnet Info-Fenster mit Geschwindigkeit und Kurs
- GPXManager: appendLocation schreibt nur noch per appendText statt Datei komplett neu zu schreiben (O(1) statt O(n))
- TrackingService: finalizeSession() beim Stoppen aufrufen, um GPX-Footer korrekt abzuschließen

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mobile/src/main/java/de/tstratmann/gpstracker/TrackDetailActivity.kt

@@ -1,6 +1,10 @@
 package de.tstratmann.gpstracker
 
+import android.graphics.Bitmap
+import android.graphics.Canvas
 import android.graphics.Color
+import android.graphics.Paint
+import android.graphics.Path
 import android.os.Bundle
 import androidx.appcompat.app.AppCompatActivity
 import androidx.core.content.ContextCompat
@@ -18,6 +22,7 @@ class TrackDetailActivity : AppCompatActivity(), OnMapReadyCallback {
     private lateinit var binding: ActivityTrackDetailBinding
     private lateinit var googleMap: GoogleMap
     private lateinit var track: Track
+    private val waypointMarkers = mutableListOf<Marker>()
 
     companion object {
         const val EXTRA_TRACK_NAME = "track_name"
@@ -86,6 +91,93 @@ class TrackDetailActivity : AppCompatActivity(), OnMapReadyCallback {
         drawTrackWithSpeedGradient()
         addStartEndMarkers()
         fitMapToTrack()
+
+        googleMap.setOnCameraIdleListener {
+            displayWaypoints(googleMap.cameraPosition.zoom)
+        }
+
+        googleMap.setOnMarkerClickListener { marker ->
+            val point = marker.tag as? TrackPoint ?: return@setOnMarkerClickListener false
+            marker.title = formatWaypointSpeed(point.speed)
+            marker.snippet = "Kurs: ${point.bearing.toInt()}°"
+            marker.showInfoWindow()
+            true
+        }
+    }
+
+    private fun getWaypointStride(zoom: Float): Int = when {
+        zoom < 12f -> Int.MAX_VALUE
+        zoom < 13f -> 100
+        zoom < 14f -> 50
+        zoom < 15f -> 20
+        zoom < 16f -> 10
+        zoom < 17f -> 5
+        zoom < 18f -> 3
+        else -> 1
+    }
+
+    private fun createArrowBitmap(bearing: Float): BitmapDescriptor {
+        val size = 48
+        val bitmap = Bitmap.createBitmap(size, size, Bitmap.Config.ARGB_8888)
+        val canvas = Canvas(bitmap)
+        val cx = size / 2f
+        val cy = size / 2f
+
+        val fillPaint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
+            color = Color.WHITE
+            style = Paint.Style.FILL
+        }
+        val strokePaint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
+            color = Color.argb(220, 30, 100, 200)
+            style = Paint.Style.STROKE
+            strokeWidth = 3f
+        }
+
+        val path = Path().apply {
+            moveTo(cx, cy - size * 0.38f)
+            lineTo(cx + size * 0.22f, cy + size * 0.30f)
+            lineTo(cx, cy + size * 0.12f)
+            lineTo(cx - size * 0.22f, cy + size * 0.30f)
+            close()
+        }
+
+        canvas.rotate(bearing, cx, cy)
+        canvas.drawPath(path, fillPaint)
+        canvas.drawPath(path, strokePaint)
+
+        return BitmapDescriptorFactory.fromBitmap(bitmap)
+    }
+
+    private fun displayWaypoints(zoom: Float) {
+        waypointMarkers.forEach { it.remove() }
+        waypointMarkers.clear()
+
+        val stride = getWaypointStride(zoom)
+        if (stride == Int.MAX_VALUE) return
+
+        track.points.forEachIndexed { index, point ->
+            if (index % stride == 0) {
+                val marker = googleMap.addMarker(
+                    MarkerOptions()
+                        .position(LatLng(point.latitude, point.longitude))
+                        .icon(createArrowBitmap(point.bearing))
+                        .anchor(0.5f, 0.5f)
+                        .flat(true)
+                ) ?: return@forEachIndexed
+                marker.tag = point
+                waypointMarkers.add(marker)
+            }
+        }
+    }
+
+    private fun formatWaypointSpeed(speedMs: Float): String {
+        val useKnots = track.activityType?.useKnots == true
+        return if (useKnots) {
+            val knots = speedMs * 1.94384f
+            "%.1f kn".format(knots)
+        } else {
+            "${(speedMs * 3.6f).toInt()} km/h"
+        }
     }
 
     private fun drawTrackWithSpeedGradient() {