feat(learn,todo,train,meditate,ui): the navigation bar flush with the foot, a fade above it

Below medium windows the frame's bar is Material 3 Expressive's again,
flush with the window's foot (decided 7 Nov 2026 by the developer): the
theme's NavigationBar across the whole width, 64 dp in its own colour,
running on beneath the system's bar as Material's does. Right above it a
fade 32 dp high, from the background of the screen in view (the
ScreenTint's, the theme's surface before a screen claims one) made
see-through to that background, so what scrolls on beneath the bar fades
out rather than being cut off; taps pass through it. The rail of wider
windows stays as it was.

halkyon_ui's FloatingNavigationBar becomes FlushNavigationBar, its side
margin, lift and rounded card gone. Its coverOf is now the whole room
content leaves at its end, the system's bar included (the body's bottom
padding: bar, fade and system's bar inside the frame, the system's bar
alone outside it), and floating buttons rise by buttonLiftOf, the same
without the system's bar, which a Scaffold keeps them clear of itself.
Lists, Todo's hour grid and the Plan's sheet keep using coverOf; the
Profile's list no longer adds the system's bar a second time.

Whats-New: The navigation bar sits at the bottom edge again, with a soft fade above it.
Whats-New-De: Die Navigationsleiste sitzt wieder am unteren Rand, mit einem sanften Übergang darüber.
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/train/halkyon_train_flutter/lib/src/tabs/profile_tab.dart

@@ -48,12 +48,12 @@ class const _Progress() extends StatelessWidget {
     child: ConstrainedBox(
       constraints: const BoxConstraints(maxWidth: readableContentMaxWidth),
       child: ListView(
-        // Room at the end for the navigation bar floating over the foot.
+        // Room at the end for the navigation bar and its fade at the foot.
         padding: EdgeInsets.fromLTRB(
           12,
           8,
           12,
-          12 + FloatingNavigationBar.coverOf(context),
+          12 + FlushNavigationBar.coverOf(context),
         ),
         children: const [
           BodyCard(),

apps/train/halkyon_train_flutter/lib/src/tabs/tab_screen.dart

@@ -34,11 +34,11 @@ class const TabScreen({
         bottom: bottom,
       ),
       body: body,
-      // Clear of the navigation bar floating over the foot on a phone.
+      // Clear of the navigation bar and its fade at the foot on a phone.
       floatingActionButton: switch (floatingActionButton) {
         final button? => Padding(
           padding: EdgeInsets.only(
-            bottom: FloatingNavigationBar.coverOf(context),
+            bottom: FlushNavigationBar.buttonLiftOf(context),
           ),
           child: button,
         ),