Make the custom accent-color picker actually retint the app

NooColors (the color source for nearly the whole redesigned UI) only
had two states: the fixed violet palette, or a Material You dynamic
scheme when "Match wallpaper" was on. The 5-swatch custom accent
picker still wrote to SettingsController.seedColor, but nothing read
it back into NooColors - it only reached the underlying Material
ColorScheme, which the Noo-styled UI doesn't use, so every custom
swatch looked identically purple.

Add NooColors.fromSeed, which retints just the accent/accentText/
accentSoft roles from a chosen seed color via ColorScheme.fromSeed
(bg/surface/fg etc. stay the fixed warm-neutral palette, unlike
dynamic color which reshapes everything to match the wallpaper), and
wire it into AppTheme.light/dark. Add a "Default" (Noo violet) swatch
to the picker, and move SettingsController's initial seed color to
that same violet so a fresh install - or any device without dynamic
color - keeps today's look instead of picking up the leftover
Nextcloud-blue default from before seed colors did anything visible.

lib/providers/settings_controller.dart

@@ -37,7 +37,7 @@ class SettingsController extends ChangeNotifier {
   final Future<SharedPreferences> _prefsFuture =
       SharedPreferences.getInstance();
 
-  Color _seedColor = AppTheme.defaultNextcloudBlue;
+  Color _seedColor = AppTheme.defaultAccent;
   ThemeMode _themeMode = ThemeMode.system;
   bool _useDynamicColor = true;
   bool _amoledDark = false;