Auto dark/bright follows the sun at the tablet's own location

Without a light sensor an app may read (the Redmi Pad SE has none), auto is dark while
the sun is more than 4° below the horizon (lib/theme/sun.dart), at Android's coarse
location, polled and kept across restarts. The tablet's own, not the Orin's GNSS: the
tablet has its own GNSS and cellular. Android's dark theme is the last fallback.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/theme/sun.dart

@@ -0,0 +1,48 @@
+// Auto dark/bright from the sun: dark once it is far enough below the horizon, at the
+// tablet's own rough location (Android's, not the Orin's: the tablet stands on its own).
+// Google Maps on this tablet does the same (it reads no light sensor; checked). Needs no
+// sensor; does not know about tunnels or car parks.
+
+import 'dart:math' as math;
+
+import 'package:flutter/services.dart';
+
+/// The tablet's newest known location, roughly, from MainActivity; null without one
+/// (or before location is granted: the first call asks).
+Future<(double, double)?> androidLocation() async {
+  try {
+    final v = await const MethodChannel('yukart/location')
+        .invokeListMethod<double>('last');
+    return v == null || v.length < 2 ? null : (v[0], v[1]);
+  } catch (_) {
+    return null; // No Android side (tests, desktop).
+  }
+}
+
+/// Dark while the sun is lower than this (degrees; sunset is -0.8, civil dusk -6).
+const darkBelowDeg = -4.0;
+
+/// The sun's elevation above the horizon, in degrees, at [lat], [lon] (degrees, east
+/// positive) and [time]. The usual low-precision formulae (the Astronomical Almanac's,
+/// as NOAA uses them): within about a degree, plenty for this.
+double sunElevation(double lat, double lon, DateTime time) {
+  double rad(double d) => d * math.pi / 180;
+  double deg(double r) => r * 180 / math.pi;
+  final d =
+      time.toUtc().millisecondsSinceEpoch / 86400000 + 2440587.5 - 2451545.0;
+  final g = rad(357.529 + 0.98560028 * d); // mean anomaly
+  final q = 280.459 + 0.98564736 * d; // mean longitude
+  final l = rad(q + 1.915 * math.sin(g) + 0.020 * math.sin(2 * g));
+  final e = rad(23.439 - 0.00000036 * d); // obliquity
+  final ra = math.atan2(math.cos(e) * math.sin(l), math.cos(l));
+  final dec = math.asin(math.sin(e) * math.sin(l));
+  final gmst = (18.697374558 + 24.06570982441908 * d) % 24;
+  final ha = rad(gmst * 15 + lon) - ra;
+  final phi = rad(lat);
+  return deg(
+    math.asin(
+      math.sin(phi) * math.sin(dec) +
+          math.cos(phi) * math.cos(dec) * math.cos(ha),
+    ),
+  );
+}