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/overlay_constants.dart

@@ -0,0 +1,37 @@
+// Overlay sizing and display constants — SCREAMING_SNAKE per project convention
+
+/// Overlay window configuration for proportional sizing and appearance
+abstract class OverlayConfig {
+  /// Overlay width as percentage of screen width
+  static const double WIDTH_PERCENTAGE = 0.45;
+
+  /// Overlay height aspect ratio relative to width
+  static const double ASPECT_RATIO = 0.6;
+
+  /// Background color opacity
+  static const double BACKGROUND_OPACITY = 0.85;
+
+  /// Border color opacity
+  static const double BORDER_OPACITY = 0.3;
+
+  /// Speed text font size as ratio of overlay width
+  static const double FONT_SIZE_RATIO = 0.2;
+
+  /// Unit text font size as ratio of speed font size
+  static const double UNIT_FONT_RATIO = 0.6;
+
+  /// Navigation icon size as ratio of overlay width
+  static const double ICON_SIZE_RATIO = 0.72;
+
+  /// Heading text font size as ratio of speed font size
+  static const double HEADING_FONT_RATIO = 0.4;
+
+  /// Default screen width for calculations when actual width unavailable
+  static const double DEFAULT_SCREEN_WIDTH = 400.0;
+
+  /// Default overlay width for calculations
+  static const double DEFAULT_OVERLAY_WIDTH = 280.0;
+
+  /// Default overlay height for calculations
+  static const double DEFAULT_OVERLAY_HEIGHT = 140.0;
+}