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>

test/palette_test.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 
 import 'package:display/theme/light.dart';
 import 'package:display/theme/palette.dart';
+import 'package:display/theme/sun.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:shared_preferences/shared_preferences.dart';
@@ -69,5 +70,57 @@ void main() {
       const Duration(seconds: 6),
     ); // no new reading: the timer ends the hold
     expect(t.isDark, isTrue);
+    t.dispose();
   });
+
+  test(
+    'the sun: high at noon, below at midnight, sets near the almanac time',
+    () {
+      // Tokyo, 2026-06-21: noon (03:00 UTC) about 77°, midnight well below.
+      expect(
+        sunElevation(35.68, 139.77, DateTime.utc(2026, 6, 21, 2, 43)),
+        closeTo(77.7, 1),
+      );
+      expect(
+        sunElevation(35.68, 139.77, DateTime.utc(2026, 6, 21, 15)),
+        lessThan(-20),
+      );
+      // Helsinki, 2026-10-15: sunset 18:14 EEST (15:14 UTC): -0.8° there.
+      expect(
+        sunElevation(60.17, 24.94, DateTime.utc(2026, 10, 15, 15, 14)),
+        closeTo(-0.8, 0.6),
+      );
+    },
+  );
+
+  test(
+    'auto: the sun before Android, the light sensor before the sun',
+    () async {
+      SharedPreferences.setMockInitialValues({});
+      final prefs = await SharedPreferences.getInstance();
+      dispatcher.platformBrightnessTestValue = Brightness.light;
+      var now = DateTime.utc(2026, 6, 21, 15); // Tokyo midnight
+      final t = Themes(
+        't',
+        Palette.fallbackDark,
+        Palette.fallbackBright,
+        prefs,
+        () => now,
+      );
+      expect((t.isDark, t.autoSource), (false, "Android's dark theme"));
+      t.setPosition(35.68, 139.77);
+      expect((t.isDark, t.autoSource), (true, 'the sun'));
+      now = DateTime.utc(2026, 6, 21, 3);
+      final again = Themes(
+        't',
+        Palette.fallbackDark,
+        Palette.fallbackBright,
+        prefs,
+        () => now,
+      );
+      expect(again.isDark, isFalse); // position kept; Tokyo noon
+      t.dispose();
+      again.dispose();
+    },
+  );
 }