feat(ui): on a phone, the floating button sits just above the navigation bar, its screen's fade under it

Each tab's screen paints the bar's fade beneath its own button
(FrameFade as its bottomNavigationBar, extendBody), so the button stands
16 pixels above the bar, as Material's, instead of above the fade; the
frame keeps its own fade for screens without one, and gets it back while
a page is pushed over the screen.

Whats-New: On a phone, the + button now sits just above the navigation bar.
Whats-New-De: Auf dem Handy sitzt der +-Knopf jetzt direkt über der Navigationsleiste.

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/tab_screen.dart

@@ -34,16 +34,13 @@ class const TabScreen({
         bottom: bottom,
       ),
       body: body,
-      // Clear of the navigation bar and its fade at the foot on a phone.
-      floatingActionButton: switch (floatingActionButton) {
-        final button? => Padding(
-          padding: EdgeInsets.only(
-            bottom: FlushNavigationBar.buttonLiftOf(context),
-          ),
-          child: button,
-        ),
-        null => null,
-      },
+      // On a phone the button stands over the fade above the navigation
+      // bar, which the screen paints beneath it, the content running on
+      // beneath both (the developer's review, 9 Nov 2026).
+      extendBody: true,
+      floatingActionButtonLocation: FrameFade.buttonLocation(context),
+      floatingActionButton: floatingActionButton,
+      bottomNavigationBar: const FrameFade(),
     ),
   );
 }