docs: add design for trail line visualization at low zoom for tracked callsigns

- Show polyline trail instead of heat map when tracking callsign at zoom ≤ 8
- Individual markers remain at zoom > 8
- Trail line connects positions chronologically
- Respects trail duration setting

docs/plans/2026-02-27-callsign-trail-line-design.md

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+# Trail Line Display for Tracked Callsigns at Low Zoom
+
+## Problem Statement
+
+When tracking a specific callsign (e.g., W5ISP-9), the map currently displays aggregated heat map data at zoom levels ≤ 8. This makes it impossible to see the individual path points and trajectory of the tracked station when zoomed out. Users need to see the complete path of a tracked callsign regardless of zoom level.
+
+## Solution Overview
+
+Implement a trail line visualization that displays when tracking a specific callsign at low zoom levels (≤ 8). Instead of showing a heat map, the system will draw a polyline connecting all the station's position points in chronological order, clearly showing the path traveled.
+
+## Behavior Specification
+
+### When Tracking a Callsign
+
+- **Zoom ≤ 8**: Display trail line (polyline) connecting all positions chronologically
+  - Clear any existing heat map
+  - Fetch all packets for tracked callsign within time threshold
+  - Sort by timestamp (chronological order)
+  - Draw polyline on map
+
+- **Zoom > 8**: Display individual markers (current behavior)
+  - Show APRS symbols at each position
+  - Enable popup interactions
+
+### When NOT Tracking a Callsign
+
+- **Zoom ≤ 8**: Display heat map (current behavior)
+- **Zoom > 8**: Display individual markers (current behavior)
+
+## Technical Design
+
+### Backend Changes (Elixir)
+
+**File: `lib/aprsme_web/live/map_live/display_manager.ex`**
+
+1. Modify `handle_zoom_threshold_crossing/2`:
+   - Check if `socket.assigns.tracked_callsign != ""`
+   - If tracking and zoom ≤ 8: call new function to send trail line
+   - If tracking and zoom > 8: show markers as normal
+   - If not tracking: use existing heat map/marker logic
+
+2. Add new function `send_trail_line_for_tracked_callsign/1`:
+   - Query all packets for tracked callsign within time threshold
+   - Sort packets by `received_at` (chronological order)
+   - Extract coordinates (lat, lng, timestamp)
+   - Push event `show_trail_line` with trail data
+
+**File: `lib/aprsme_web/live/map_live/index.ex`**
+
+3. Update bounds update logic:
+   - When processing bounds updates at zoom ≤ 8 with tracked callsign
+   - Send trail line instead of heat map
+
+### Frontend Changes (TypeScript)
+
+**File: `assets/js/map.ts`**
+
+1. Add state tracking:
+   - `currentTrailLine: Polyline | null` - store reference to current trail line
+
+2. Add event handler `show_trail_line`:
+   - Clear existing trail line if present
+   - Create Leaflet Polyline from coordinates
+   - Style the polyline (color, weight, opacity)
+   - Add to map
+   - Store reference in state
+
+3. Add event handler `clear_trail_line`:
+   - Remove trail line from map
+   - Clear state reference
+
+4. Update existing handlers:
+   - `show_heat_map`: Clear trail line before showing heat map
+   - `show_markers`: Clear trail line before showing markers
+   - `clear_all_markers`: Also clear trail line
+
+### Data Structure
+
+**Trail Line Event Payload:**
+```typescript
+{
+  callsign: string,
+  points: Array<{
+    lat: number,
+    lng: number,
+    timestamp: string
+  }>
+}
+```
+
+### Visual Styling
+
+**Trail Line Appearance:**
+- Color: `#3B82F6` (blue-500) - distinct from other map elements
+- Weight: 3 pixels - visible but not overwhelming
+- Opacity: 0.8 - allows seeing map features underneath
+- Line Cap: round - smoother appearance at endpoints
+- Line Join: round - smoother corners
+- Interactive: true - allow clicking for info
+- Dash Pattern: none (solid line)
+
+**Optional Enhancement (Future):**
+- Direction arrows along the line
+- Gradient color showing time progression
+- Click on line to show timestamp at that point
+
+## Implementation Notes
+
+### Time Threshold Handling
+- Use existing `socket.assigns.packet_age_threshold` for filtering
+- Trail line respects the "Trail Duration" setting (1 hour, 6 hours, etc.)
+- When trail duration changes, regenerate trail line
+
+### Performance Considerations
+- Trail lines are only generated for tracked callsigns (not all stations)
+- Polylines are efficient for rendering many points
+- Clear trail line when switching away from tracked callsign
+- Debounce trail line updates during rapid zoom changes
+
+### Edge Cases
+- Empty trail (no packets): Don't draw anything
+- Single point: Don't draw line (show marker instead)
+- Zoom threshold crossing: Smoothly transition between trail and markers
+- Callsign tracking cleared: Remove trail line immediately
+
+## Testing Strategy
+
+1. **Manual Testing:**
+   - Track a callsign and zoom out to ≤ 8: verify trail line appears
+   - Zoom in to > 8: verify markers appear
+   - Clear tracking: verify trail line disappears
+   - Change trail duration: verify trail line updates
+
+2. **Visual Testing:**
+   - Verify trail line color and styling
+   - Check line smoothness and appearance
+   - Ensure map features remain visible underneath
+
+3. **Edge Cases:**
+   - Callsign with no packets
+   - Callsign with single packet
+   - Callsign with very long trail (performance)
+   - International date line crossing
+
+## Future Enhancements
+
+1. **Direction Indicators**: Add small arrows along the trail to show direction of travel
+2. **Time-based Coloring**: Gradient color from old (gray) to new (bright blue)
+3. **Interactive Points**: Click on trail to see packet details at that position
+4. **Trail Simplification**: Use line simplification algorithm (Douglas-Peucker) for very long trails
+5. **Multiple Callsigns**: Support tracking multiple callsigns with different colored trails