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.

test/widgets/noo/core_test.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
+import 'package:noo/theme/app_theme.dart';
 import 'package:noo/theme/design_tokens.dart';
 import 'package:noo/widgets/noo/core/noo_avatar.dart';
 import 'package:noo/widgets/noo/core/noo_badge.dart';
@@ -17,6 +18,45 @@ import 'noo_test_utils.dart';
 void main() {
   setUpNooTests();
 
+  group('NooColors.fromSeed', () {
+    test('the default accent returns the untouched fixed palette', () {
+      expect(
+        identical(
+          NooColors.fromSeed(AppTheme.defaultAccent, Brightness.light),
+          NooColors.light,
+        ),
+        isTrue,
+      );
+      expect(
+        identical(
+          NooColors.fromSeed(AppTheme.defaultAccent, Brightness.dark),
+          NooColors.dark,
+        ),
+        isTrue,
+      );
+    });
+
+    test('a custom seed retints only the accent roles', () {
+      const seed = Color(0xFF009688); // Ocean Teal
+      final light = NooColors.fromSeed(seed, Brightness.light);
+      expect(light.accent, seed);
+      expect(light.accentText, seed);
+      expect(light.accentSoft, isNot(NooColors.light.accentSoft));
+      // The rest of the palette is untouched - a named accent choice
+      // retints the brand color, it doesn't reshape the surface.
+      expect(light.bg, NooColors.light.bg);
+      expect(light.surface, NooColors.light.surface);
+      expect(light.fg1, NooColors.light.fg1);
+
+      final dark = NooColors.fromSeed(seed, Brightness.dark);
+      expect(dark.accent, seed);
+      // Dark accentText is a lighter/desaturated tint for contrast, not
+      // the raw seed itself.
+      expect(dark.accentText, isNot(seed));
+      expect(dark.bg, NooColors.dark.bg);
+    });
+  });
+
   group('NooAvatar', () {
     testNooWidgets('current user uses accent-soft/accent-text', (
       tester,

test/widgets/noo/noo_test_utils.dart

@@ -15,10 +15,13 @@ void setUpNooTests() {
 /// Themes built through the real [AppTheme] (non-dynamic), so the
 /// [NooColors] extension is registered exactly as in the app. Built lazily
 /// inside test bodies - AppTheme touches Google Fonts, which needs the test
-/// binding to exist first.
+/// binding to exist first. Seeded with [AppTheme.defaultAccent] (not just
+/// any color) so component tests exercise the exact fixed [NooColors.light]/
+/// `.dark` palette their color assertions are written against, rather than
+/// a [NooColors.fromSeed] retint - see its doc comment.
 ThemeData nooTheme(Brightness brightness) => brightness == Brightness.light
-    ? AppTheme.light(AppTheme.defaultNextcloudBlue, useDynamicColor: false)
-    : AppTheme.dark(AppTheme.defaultNextcloudBlue, useDynamicColor: false);
+    ? AppTheme.light(AppTheme.defaultAccent, useDynamicColor: false)
+    : AppTheme.dark(AppTheme.defaultAccent, useDynamicColor: false);
 
 Future<void> pumpNoo(
   WidgetTester tester,
@@ -43,7 +46,8 @@ Future<void> pumpNoo(
 /// Runs [body] once per theme, each as its own named test.
 void testNooWidgets(
   String description,
-  Future<void> Function(WidgetTester tester, ThemeData theme, NooColors colors) body,
+  Future<void> Function(WidgetTester tester, ThemeData theme, NooColors colors)
+  body,
 ) {
   for (final brightness in Brightness.values) {
     testWidgets('$description (${brightness.name})', (tester) async {
@@ -59,7 +63,10 @@ BoxDecoration decorationOf(WidgetTester tester, Finder finder) {
   final boxes = find.ancestor(
     of: finder,
     matching: find.byWidgetPredicate(
-      (w) => w is DecoratedBox && w.decoration is BoxDecoration && (w.decoration as BoxDecoration).color != null,
+      (w) =>
+          w is DecoratedBox &&
+          w.decoration is BoxDecoration &&
+          (w.decoration as BoxDecoration).color != null,
     ),
     matchRoot: true,
   );