feat(ui,learn,todo,train,meditate): a wave in the app's colour over the navigation bar

The developer's choice of 7 Nov 2026 (the mockup's variant E): a tap on
another destination sends a radial glow in the app's colour over the bar
from that destination's centre, solid to 35 % of its radius and gone at
70 %, growing to cover the bar (cubic(.2, .7, .2, 1)) while it fades from
half its strength in dark mode and a third in light, over 1.2 s, behind
the destinations and clipped to the bar. None for the destination already
chosen, none with animations off, and none for a tab changed without a
tap (a link, back): the wave answers the finger. The destination chosen
takes the app's colour, its indicator tinted 22 % with it, without
Material's ripple or the indicator growing; the bar's top corners are
rounded, 20 dp, the screen showing through them.

Each app gives the frame its colour (NavigationFrame's accent, from
PaletteColor): Learn's purple, its icon's last colour, Todo's strawberry,
Train's carrot and Meditate's fern, their screens' wash.

For trying it on a phone, a debug build's Settings has a section
Navigation bar (debug), halkyon_ui's NavigationBarDebugSection, switching
live, in memory alone: the variant (A Material only, C the wave with
Material's fill, E without it, F without the indicator too), the wave's
time (0.7 to 2 s) and the corners (0 to 28 dp). Release builds never read
it and show shippingNavigationBarStyle, the debug section's start: to
remove it, delete the section from the four Settings screens and
debugNavigationBarStyle, and let effectiveNavigationBarStyle return the
shipping style.

Whats-New: Tapping a tab sends a soft wave in the app's colour across the navigation bar, now with rounded corners.
Whats-New-De: Ein Tipp auf einen Tab schickt eine sanfte Welle in der Farbe der App über die Navigationsleiste, jetzt mit abgerundeten Ecken.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/lib/src/settings/settings_screen.dart

@@ -105,6 +105,9 @@ class const SettingsScreen({super.key}) extends StatelessWidget {
                       ],
                     ),
                   ),
+                  // The navigation bar's looks to try on a phone, in debug
+                  // builds alone (halkyon_ui's).
+                  const NavigationBarDebugSection(),
                   const SizedBox(height: 12),
                   SectionCard(
                     padding: const EdgeInsets.symmetric(vertical: 8),

apps/todo/halkyon_todo_flutter/lib/src/tabs/todo_shell.dart

@@ -52,6 +52,8 @@ class _TodoShellState() extends State<TodoShell> {
           child: NavigationFrame(
             appName: l10n.appTitle,
             icon: todoIcon,
+            // The bar's wave and chosen tab: Todo's strawberry, its screens' wash.
+            accent: PaletteColor.strawberry.background,
             selectedIndex: shell.currentIndex,
             hidesNavigationBar: _selection.active || _hold.held,
             // A tab chosen again goes back to its first screen; choosing tasks