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/timing_constants.dart

@@ -0,0 +1,16 @@
+// Timer and interval constants — SCREAMING_SNAKE per project convention
+
+/// Timing configuration for background tasks and UI interactions
+abstract class TimingConfig {
+  /// Interval for background heartbeat to keep GPS alive
+  static const Duration HEARTBEAT_INTERVAL = Duration(seconds: 5);
+
+  /// Interval for checking overlay window status
+  static const Duration OVERLAY_STATUS_CHECK_INTERVAL = Duration(milliseconds: 1000);
+
+  /// Delay before closing overlay after tap
+  static const Duration TAP_CLOSE_DELAY = Duration(milliseconds: 500);
+
+  /// Delay for long press gesture signal
+  static const Duration LONG_PRESS_SIGNAL_DELAY = Duration(milliseconds: 50);
+}