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/learn/halkyon_learn_flutter/test/core/router/app_shell_test.dart

@@ -261,10 +261,12 @@ void main() {
     final pill = tester.getRect(find.byKey(FlushNavigationBar.fadeKey));
     expect(tester.getBottomLeft(find.byType(LibraryScreen)).dy, 900);
     // Before scrolling, which slides it away.
+    // Over the fade, which the Library paints beneath it, 16 above the bar
+    // (the developer's review, 9 Nov 2026).
     expect(
       tester.getRect(find.byTooltip('Create studyset')).bottom,
-      lessThan(pill.top),
-      reason: "the Library's button stands clear of the bar",
+      tester.getRect(find.byKey(FlushNavigationBar.barKey)).top - 16,
+      reason: "the Library's button stands 16 above the bar",
     );
     await tester.fling(
       find.byType(CustomScrollView),