UI-04: customizable, persisted HUD telemetry widget layout

Adds the full drag/resize/visibility infrastructure for a rider-owned HUD
layout: HudMetric (the stable 8-metric set), HudWidgetLayout (fractional
x/y/width/height + visible, with clamping to a legibility floor/ceiling
and JSON round-trip that degrades to sane defaults rather than crashing),
Config.hudLayout persistence, and a HudLayoutController that stays
in-memory-authoritative during an edit session and only writes through on
persist() -- never per drag frame.

DraggableResizableHudWidget and HudEditOverlay assemble the interaction:
edit mode is entered by a long-press on empty HUD space (not a specific
widget) and exited via Done or a tap on empty space; only while editing
does a widget attach any drag/resize gesture at all, so a normal tap can
never move one mid-ride by construction, not by an internal flag.

Fixes a real gesture-arena bug found during testing: outside edit mode, a
long-press landing on a widget was free to bubble to the overlay's
background long-press handler and wrongly enter edit mode. An inner no-op
GestureDetector of the same gesture type now absorbs it.

Adds a "Live HUD stats" Settings section, one switch per metric, that
persists immediately (unlike drag frames). Placed at the end of the
Settings list rather than in the middle -- inserting mid-list pushed
every later section below several existing tests' viewport assumptions.

Verified end-to-end on a real emulator including a full process restart:
resized a widget, force-stopped the app, relaunched, and the resize held.
No consuming screen exists yet (UI-05's job) -- verified via a throwaway
preview entry point, deleted after use.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Xki7YAcc2TiN2PRZJ2tXr

lib/src/config/config.dart

@@ -4,12 +4,15 @@
 /// recording must work with no server at all, and uploading is opt-in.
 library;
 
+import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 
 import 'package:shared_preferences/shared_preferences.dart';
 
 import '../domain/models.dart' show UnitSystem;
+import '../hud/hud_metric.dart';
+import '../hud/hud_widget_layout.dart';
 
 const _keyEndpoint = 'upload_endpoint';
 const _keyDeviceId = 'device_id';
@@ -17,6 +20,7 @@ const _keyMapEnabled = 'map_enabled';
 const _keyUnitSystem = 'unit_system';
 const _keyMountedMode = 'mounted_mode';
 const _keyCrashReportingEnabled = 'crash_reporting_enabled';
+const _keyHudLayout = 'hud_layout';
 
 /// Countries that did not adopt metric for everyday distances. Not exhaustive — a
 /// best-effort default, not a claim of authority. Anyone can override it in Settings.
@@ -88,6 +92,33 @@ class Config {
   Future<void> setCrashReportingEnabled(bool enabled) =>
       _prefs.setBool(_keyCrashReportingEnabled, enabled);
 
+  /// UI-04: every metric's position, size, and visibility on the customizable HUD.
+  /// Missing entirely (fresh install) or missing a specific metric (an app update
+  /// that added one) both fall back to [HudWidgetLayout.defaultFor] -- never a crash,
+  /// same discipline [HudWidgetLayout.fromJson] itself follows for a malformed entry.
+  Map<HudMetric, HudWidgetLayout> get hudLayout {
+    final raw = _prefs.getString(_keyHudLayout);
+    Map<String, dynamic> stored = const {};
+    if (raw != null) {
+      try {
+        stored = jsonDecode(raw) as Map<String, dynamic>;
+      } catch (_) {
+        stored = const {};
+      }
+    }
+    return {
+      for (final metric in HudMetric.values)
+        metric: stored[metric.name] is Map<String, dynamic>
+            ? HudWidgetLayout.fromJson(metric, stored[metric.name] as Map<String, dynamic>)
+            : HudWidgetLayout.defaultFor(metric),
+    };
+  }
+
+  Future<void> setHudLayout(Map<HudMetric, HudWidgetLayout> layout) => _prefs.setString(
+    _keyHudLayout,
+    jsonEncode({for (final entry in layout.entries) entry.key.name: entry.value.toJson()}),
+  );
+
   /// Stable per-install id so a server can distinguish riders in a group.
   String get deviceId {
     final existing = _prefs.getString(_keyDeviceId);