fix(ui): keep pushed screens clear of the Android navigation bar

Plezy is edge-to-edge on Android whether it asks to be or not: targetSdk is
36, Android 15 enforces edge-to-edge for apps targeting 35+, and Android 16
disables the windowOptOutEdgeToEdgeEnforcement escape hatch. The only
SystemUiMode.edgeToEdge call in the app fires on video-player exit, so on
API 35+ the window is edge-to-edge from the first frame and
MediaQuery.padding.bottom is a real ~48dp overlap under 3-button navigation.

MainScreen's phone layout hides that. It supplies a bottomNavigationBar and
never sets extendBody, so Flutter's Scaffold strips padding.bottom from the
body MediaQuery and every tab is already safe. Routes pushed on the profile
navigator are full-screen siblings of MainScreen with no bottom bar, so they
receive the untouched inset and nothing consumes it - the last settings card
and the final log lines render under the back, home, and recents buttons.

Three shared hosts own most of those routes, so the inset is consumed there:
FocusedScrollScaffold (25 screens, counting the SettingsPage wrapper) and
FocusableDetailScreenMixin.buildDetailScaffold (4) now append a trailing
SliverSystemBottomInset, and the four screens that build their own Scaffold
around a CustomScrollView append it directly.

The new widget codifies the convention this repository had already written
down but open-coded - insets baked into the scroll content rather than a
SafeArea around the scroll view - so content still paints under the bar while
the scroll extent grows enough to bring the last row above it. It reads
padding from its own context and collapses to zero height wherever the inset
is already zero: desktop, Android TV, tvOS via _AppleTvScale, and inside
MainScreen's tab bodies. No platform branching, and it stacks additively with
the music detail screens' existing mini-player spacers, which is correct
because the mini-player itself floats above the navigation bar on a pushed
route.

Scroll views that are not sliver lists take the inset in their own padding:
the companion remote's ListView, the auth screen's scroll container, and the
two SliverFillRemaining sign-in forms, whose children size themselves from
the extent remaining before them and so cannot be helped by a trailing
sliver. The logs empty state is left alone for the same reason inverted - it
already fills the viewport, and a trailing inset would only add scroll slack.

Verified on a Pixel 7 running Android 16 (API 36) with 3-button navigation:
Settings, Logs, and Video Playback all end clear of the bar.

close #1766

lib/screens/settings/add_plex_account_screen.dart

@@ -138,7 +138,7 @@ class _AddPlexAccountScreenState extends State<AddPlexAccountScreen> with AsyncF
         SliverFillRemaining(
           hasScrollBody: false,
           child: Padding(
-            padding: const EdgeInsets.all(24),
+            padding: .fromLTRB(24, 24, 24, 24 + MediaQuery.paddingOf(context).bottom),
             child: Center(
               child: ConstrainedBox(
                 constraints: const BoxConstraints(maxWidth: 420),