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

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+// 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;
+}

lib/config/overlay_constants.dart

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+// 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;
+}

lib/config/timing_constants.dart

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+// 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);
+}