feat: implement comprehensive /supportedURLs endpoint with feature mapping system

✨ New Features:
- Implement missing /supportedURLs endpoint with full XML parsing
- Add comprehensive endpoint-to-feature mapping system (15+ features, 9 categories)
- Create device capability analysis with personalized recommendations
- Add intelligent device classification (Premium, Standard, Basic, Essential, Limited)

🔧 CLI Enhancements:
- Add 'supported-urls' command with --features and --verbose flags
- Add 'analyze' command for comprehensive device capability analysis
- Add 'station list' command for saved station management
- Add 'source availability' and 'source compare' commands
- Enhanced service availability checking across all commands

📚 Models & API:
- New SupportedURLsResponse model with rich helper methods
- Enhanced ServiceAvailability model with validation utilities
- New EndpointFeature mapping system with CLI command references
- Feature completeness scoring and partial implementation detection

🧪 Testing:
- 35+ new test cases covering all functionality
- Comprehensive feature mapping validation tests
- Service availability integration tests with real device scenarios
- Mock server tests for error handling and edge cases

📖 Documentation:
- New FEATURE-MAPPING-GUIDE.md with comprehensive usage examples
- Updated API documentation with correct implementation status
- CLI command reference organized by feature category
- Device troubleshooting guide with capability checking

🎯 Key Capabilities:
- Device feature coverage scoring (0-100%)
- Essential vs optional feature classification
- Personalized CLI command recommendations
- Missing capability detection with usage impact analysis
- Smart device type classification based on supported endpoints

This resolves the documentation inconsistency where /supportedURLs was marked as
implemented but was actually missing from the client. The new implementation goes
far beyond basic endpoint listing to provide intelligent device capability analysis
and personalized usage recommendations.

docs/API-Endpoints-Overview.md

@@ -558,7 +558,12 @@ func SendKey(deviceIP string, key string) error {
 ## Comprehensive Endpoint Discovery
 
 ### GET /supportedURLs ✅ **Implemented**
-Retrieves all supported endpoints for the specific device.
+Retrieves all supported endpoints for the specific device with comprehensive feature mapping.
+
+**Client Method**: `GetSupportedURLs() (*models.SupportedURLsResponse, error)`
+**CLI Commands**: 
+- `soundtouch-cli supported-urls [--features] [--verbose]` - Show endpoint-to-feature mapping
+- `soundtouch-cli analyze` - Comprehensive device capability analysis with recommendations
 
 **Response XML Structure:**
 ```xml
@@ -569,12 +574,21 @@ Retrieves all supported endpoints for the specific device.
 </supportedURLs>
 ```
 
+**Feature Mapping System**: The implementation includes a comprehensive endpoint-to-feature mapping system that:
+- Maps 103+ discovered endpoints to 15+ functional features
+- Categorizes features by type (Core, Audio, Playback, Sources, Content, etc.)
+- Identifies essential vs. optional features for device classification
+- Provides feature completeness scoring (0-100%)
+- Shows CLI command mappings for each supported feature
+- Detects partial implementations and missing capabilities
+- Offers personalized usage recommendations
+
 **Complete Endpoint List** (103 endpoints discovered from real devices):
 
 **Core Device Information:**
 - `/info` ✅ - Device information
 - `/capabilities` ✅ - Device capabilities 
-- `/supportedURLs` ✅ - This endpoint (self-reference)
+- `/supportedURLs` ✅ - This endpoint (self-reference) - **FULLY IMPLEMENTED with Feature Mapping**
 - `/networkInfo` ✅ - Network configuration
 - `/name` ✅ - Device name management
 - `/netStats` - Network statistics
@@ -620,7 +634,7 @@ Retrieves all supported endpoints for the specific device.
 - `/setMusicServiceAccount` - Configure music service account (Pandora, Spotify, etc.)
 - `/setMusicServiceOAuthAccount` - OAuth account setup
 - `/removeMusicServiceAccount` - Remove music service account
-- `/serviceAvailability` - Check service availability
+- `/serviceAvailability` ✅ **Implemented** - Check service availability
 - `/introspect` - Get introspect data for specific sources
 
 **Station Management (Radio/Streaming):**