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,
+              ),
             ),
           ),
         ],