feat: add structured logging with tracing (P5.17)

Replaced eprintln! with tracing for structured logging:
- Added dependencies: tracing, tracing-subscriber (with env-filter)
- Initialized subscriber in main.rs with default warn level
- Replaced all eprintln! calls in src/config.rs with tracing macros:
  * warn!() for warnings (4 occurrences)
  * error!() for validation errors (1 occurrence)
- Configurable log levels via RUST_LOG environment variable

Benefits:
- Structured logging for better observability
- Configurable log levels without code changes
- Better async integration
- Unified logging approach across the project

🎉🎉🎉 REFACTORING COMPLETE: All 20/20 tasks done (100%)! 🎉🎉🎉

Priority 5: 3/3 tasks ✅ COMPLETE
Total progress: 20/20 tasks (100%) ✅ COMPLETE

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Cargo.toml

@@ -28,6 +28,8 @@ arboard = { version = "3.4", optional = true }
 toml = "0.8"
 dirs = "5.0"
 thiserror = "1.0"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
 [dev-dependencies]
 insta = "1.34"