refactor(data): create layered architecture with core/data/ui packages

- Move constants from lib/config/ to lib/core/constants/
- Move color themes to lib/core/theme/
- Create LocationData domain model in data/models/
- Create lightweight GpsService in data/services/
- Create LocationRepository in data/repositories/ consolidating
  GpsDataManager business logic (GPS stream, staleness, precision)
- Create shared SpeedDisplay widget in ui/shared/
- Defer overlay feature — stubs reserved in ui/features/overlay/

lib/core/constants/gps_constants.dart

@@ -0,0 +1,31 @@
+// 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;
+
+  /// Speed threshold for switching to HIGH precision (m/s) - 12 km/h
+  static const double PRECISION_UP_THRESHOLD_MPS = 3.33;
+
+  /// Speed threshold for switching to BALANCED precision (m/s) - 8 km/h
+  static const double PRECISION_DOWN_THRESHOLD_MPS = 2.22;
+
+  /// Minimum time after first fix before precision switching starts
+  static const Duration PRECISION_SWITCH_FIRST_FIX_DELAY = Duration(seconds: 5);
+}

lib/core/constants/overlay_constants.dart

@@ -0,0 +1,49 @@
+// 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;
+
+  /// Time after which overlay dims speed display due to stale data
+  static const Duration STALENESS_DIM_THRESHOLD = Duration(seconds: 3);
+
+  /// Time after which overlay shows dashes instead of speed
+  static const Duration STALENESS_DASH_THRESHOLD = Duration(seconds: 10);
+
+  /// Opacity value for dimmed stale state
+  static const double STALENESS_DIM_OPACITY = 0.5;
+
+  /// Interval for checking data staleness
+  static const Duration STALENESS_CHECK_INTERVAL = Duration(seconds: 1);
+}

lib/core/constants/speed_unit.dart

@@ -0,0 +1,20 @@
+enum SpeedUnit {
+  kmh('km/h', 1.0),
+  mph('mph', 0.621371),
+  knots('kts', 0.539957);
+
+  const SpeedUnit(this.label, this.multiplier);
+
+  final String label;
+  final double multiplier;
+
+  double convert(double speedMps) {
+    return speedMps * 3.6 * multiplier;
+  }
+
+  SpeedUnit get next {
+    final values = SpeedUnit.values;
+    final currentIndex = values.indexOf(this);
+    return values[(currentIndex + 1) % values.length];
+  }
+}

lib/core/constants/timing_constants.dart

@@ -0,0 +1,23 @@
+// 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);
+
+  /// GPS grace period - continues for 30s after overlay close when backgrounded
+  /// DEPRECATED: Replaced by BACKGROUND_GRACE_PERIOD (7s) for lifecycle management
+  static const Duration GPS_GRACE_PERIOD = Duration(seconds: 30);
+
+  /// Background grace period - time before GPS stops when app backgrounded without overlay
+  static const Duration BACKGROUND_GRACE_PERIOD = Duration(seconds: 7);
+}

lib/core/theme/color_themes.dart

@@ -0,0 +1,64 @@
+import 'package:flutter/material.dart';
+
+class ColorTheme {
+  final Color background;
+  final Color speedText;
+  final Color speedTextSub;
+  final Color headingText;
+  final Color unitText;
+  final String name;
+
+  const ColorTheme({
+    required this.background,
+    required this.speedText,
+    required this.speedTextSub,
+    required this.headingText,
+    required this.unitText,
+    required this.name,
+  });
+}
+
+class ColorThemes {
+  static const List<ColorTheme> themes = [
+    ColorTheme(
+      name: 'Yuzuki',
+      background: Color.fromARGB(255, 114, 85, 109),
+      speedText: Color.fromARGB(255, 246, 224, 255),
+      speedTextSub: Color.fromARGB(255, 245, 217, 245),
+      headingText: Color.fromARGB(255, 244, 201, 255),
+      unitText: Color.fromARGB(255, 244, 201, 255),
+    ),
+    ColorTheme(
+      name: 'Yuuparo',
+      background: Color.fromARGB(255, 85, 99, 112),
+      speedText: Color.fromARGB(255, 255, 171, 16),
+      speedTextSub: Color.fromARGB(255, 245, 204, 116),
+      headingText: Color.fromARGB(255, 228, 235, 238),
+      unitText: Color.fromARGB(255, 141, 196, 184),
+    ),
+    ColorTheme(
+      name: 'Yuzuki Light',
+      background: Color.fromARGB(255, 246, 224, 255),
+      speedText: Color.fromARGB(255, 114, 85, 109),
+      speedTextSub: Color.fromARGB(255, 161, 138, 170),
+      headingText: Color.fromARGB(255, 140, 94, 151),
+      unitText: Color.fromARGB(255, 114, 85, 109),
+    ),
+    ColorTheme(
+      name: 'Yuuparo Light',
+      background: Color.fromARGB(255, 228, 235, 238),
+      speedText: Color.fromARGB(255, 85, 99, 112),
+      speedTextSub: Color.fromARGB(255, 100, 100, 131),
+      headingText: Color.fromARGB(255, 221, 118, 0),
+      unitText: Color.fromARGB(255, 98, 124, 118),
+    ),
+  ];
+
+  static ColorTheme getTheme(int index) {
+    return themes[index % themes.length];
+  }
+
+  static int getNextThemeIndex(int currentIndex) {
+    return (currentIndex + 1) % themes.length;
+  }
+}