Implement /now_playing and /sources endpoints with real device integration

## New Endpoints

### GET /now_playing ✅
- Rich XML models with PlayStatus, ShuffleSetting, RepeatSetting enums
- Comprehensive playback information (track, artist, album, artwork, position)
- Device capabilities (skip, seek, favorite functionality)
- Smart display methods for different content types (music vs radio)
- Duration formatting with position/total time display

### GET /sources ✅
- Complete audio source management with SourceStatus enum
- Source categorization (Local/Remote, Streaming, Multiroom support)
- Multiple account support (multiple Spotify accounts per device)
- Availability filtering (Ready vs Unavailable sources)
- Helper methods for quick capability checks

## Real Device Integration

- Fetched actual XML responses from SoundTouch devices (192.168.178.28 & 192.168.178.35)
- Updated all test fixtures with real device data (anonymized)
- Enhanced XML models to handle all real-world fields and edge cases
- Verified compatibility across different device types and configurations

## Enhanced CLI Tool

- Added -nowplaying command with rich formatted output
- Added -sources command with categorized source listing
- Display enhancements: duration info, capabilities, source attributes
- Improved build process to use ./build/ directory consistently

## Comprehensive Testing

- 15+ unit tests for XML models with enum validation
- Client integration tests with mock HTTP responses
- Real device response validation
- Edge case handling (empty states, network errors, invalid data)

## Documentation & Guidelines

- Updated CLAUDE.md with build directory and real device testing guidelines
- Enhanced README with comprehensive usage examples
- Updated PLAN.md to reflect implementation progress
- All examples use real device data patterns

## Quality Improvements

- Type-safe XML unmarshaling with custom validation
- Consistent error handling across all endpoints
- Privacy protection (anonymized account information)
- Production-ready code structure and patterns

Features:
✅ GET /info - Device information
✅ GET /now_playing - Current playback status with full metadata
✅ GET /sources - Available audio sources with smart categorization
✅ UPnP device discovery
✅ Cross-platform CLI tool with rich output formatting
✅ Comprehensive test coverage with real device data
✅ Build automation with proper directory structure

pkg/client/client.go

@@ -76,6 +76,26 @@ func (c *Client) GetDeviceInfo() (*models.DeviceInfo, error) {
 	return &deviceInfo, nil
 }
 
+// GetNowPlaying retrieves current playback information from the /now_playing endpoint
+func (c *Client) GetNowPlaying() (*models.NowPlaying, error) {
+	var nowPlaying models.NowPlaying
+	err := c.get("/now_playing", &nowPlaying)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get now playing: %w", err)
+	}
+	return &nowPlaying, nil
+}
+
+// GetSources retrieves available audio sources from the /sources endpoint
+func (c *Client) GetSources() (*models.Sources, error) {
+	var sources models.Sources
+	err := c.get("/sources", &sources)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get sources: %w", err)
+	}
+	return &sources, nil
+}
+
 // Ping checks if the device is reachable by calling /info
 func (c *Client) Ping() error {
 	_, err := c.GetDeviceInfo()