Dark/bright: Auto (Android's dark theme), Dark or Bright, kept across restarts

Settings has a segmented Auto / Dark / Bright. Auto follows platformBrightness, so
Android's schedule too. The Dash's quick button holds the other variant; a long press
is auto again. The mode is kept with shared_preferences.

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

lib/pages/dash.dart

@@ -164,7 +164,7 @@ class DashPage extends StatelessWidget {
   }
 }
 
-/// The dash's own thin top line: the menu, link state, and the dark/bright switch.
+/// The dash's own thin top line: the menu, link state, and the dark/bright quick switch.
 class _DashBar extends StatelessWidget {
   const _DashBar({required this.boundary, required this.themes});
   final Boundary boundary;
@@ -185,12 +185,18 @@ class _DashBar extends StatelessWidget {
           MenuButton(color: p.subdued),
           Text(text, style: TextStyle(color: color, letterSpacing: 2)),
           const Spacer(),
-          IconButton(
-            tooltip: themes.isDark ? 'Bright' : 'Dark',
-            onPressed: themes.toggle,
-            icon: Icon(
-              themes.isDark ? Icons.light_mode : Icons.dark_mode,
-              color: p.subdued,
+          // Tap: hold the other variant. Long press: back to auto (Settings has both).
+          GestureDetector(
+            onLongPress: () => themes.mode = VariantMode.auto,
+            child: IconButton(
+              tooltip: themes.isDark ? 'Bright' : 'Dark',
+              onPressed: themes.toggle,
+              icon: Icon(
+                themes.mode == VariantMode.auto
+                    ? Icons.brightness_auto
+                    : (themes.isDark ? Icons.light_mode : Icons.dark_mode),
+                color: p.subdued,
+              ),
             ),
           ),
         ],

lib/pages/settings.dart

@@ -52,14 +52,38 @@ class SettingsPage extends StatelessWidget {
               onTap: () => editUrl(context, boundary, usb),
             ),
             section('Display'),
-            SwitchListTile(
-              secondary: Icon(
-                themes.isDark ? Icons.dark_mode : Icons.light_mode,
+            ListTile(
+              leading: Icon(themes.isDark ? Icons.dark_mode : Icons.light_mode),
+              title: const Text('Dark or bright'),
+              subtitle: const Text(
+                "Auto follows Android's dark theme and its schedule. "
+                "The Dash's button holds one; a long press there is auto again.",
+              ),
+            ),
+            Padding(
+              padding: const EdgeInsets.fromLTRB(72, 0, 16, 8),
+              child: SegmentedButton<VariantMode>(
+                segments: const [
+                  ButtonSegment(
+                    value: VariantMode.auto,
+                    icon: Icon(Icons.brightness_auto),
+                    label: Text('Auto'),
+                  ),
+                  ButtonSegment(
+                    value: VariantMode.dark,
+                    icon: Icon(Icons.dark_mode),
+                    label: Text('Dark'),
+                  ),
+                  ButtonSegment(
+                    value: VariantMode.bright,
+                    icon: Icon(Icons.light_mode),
+                    label: Text('Bright'),
+                  ),
+                ],
+                selected: {themes.mode},
+                showSelectedIcon: false,
+                onSelectionChanged: (s) => themes.mode = s.single,
               ),
-              title: const Text('Dark variant'),
-              subtitle: const Text('Bright for daylight, dark for night'),
-              value: themes.isDark,
-              onChanged: (v) => themes.isDark = v,
             ),
             ListTile(
               leading: const Icon(Icons.palette),