chore(01-01): create directory structure and infrastructure files

- Create lib/screens/, lib/widgets/, lib/providers/, lib/models/, lib/config/ directories
- Add Logger utility with error/warn/info/debug levels (kDebugMode guarded)
- Add GpsConfig constants (timeouts, heading validation)
- Add TimingConfig constants (heartbeat, overlay intervals)
- Add OverlayConfig constants (sizing, opacity ratios)

lib/config/gps_constants.dart

@@ -0,0 +1,22 @@
+// GPS behavioral constants — SCREAMING_SNAKE per project convention
+
+/// GPS configuration constants for data validation and timeouts
+abstract class GpsConfig {
+  /// Time threshold after which GPS data is considered stale
+  static const Duration STALE_DATA_THRESHOLD = Duration(seconds: 4);
+
+  /// Timeout for initial GPS fix acquisition (outdoor/cold start)
+  static const Duration INITIAL_FIX_TIMEOUT = Duration(seconds: 20);
+
+  /// Timeout for ongoing GPS updates (indoor/signal loss detection)
+  static const Duration UPDATE_TIMEOUT = Duration(seconds: 2);
+
+  /// Minimum valid heading value in degrees
+  static const double VALID_HEADING_MIN = 0.0;
+
+  /// Maximum valid heading value in degrees
+  static const double VALID_HEADING_MAX = 360.0;
+
+  /// Speed threshold below which heading is considered unreliable (m/s)
+  static const double LOW_SPEED_THRESHOLD = 1.0;
+}