feat: adopt smolmail protocol 1.1, adaptive nav shell, and server mail retention

lib/presentation/screens/home_screen.dart

@@ -0,0 +1,86 @@
+import "package:auto_route/auto_route.dart";
+import "package:flutter/material.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+
+import "package:smol_mail/data/providers/providers.dart";
+import "package:smol_mail/presentation/routes/app_router.gr.dart";
+import "package:smol_mail/presentation/theme/app_colors.dart";
+import "package:smol_mail/presentation/theme/breakpoints.dart";
+import "package:smol_mail/shared/configs/flash_mail_icons.dart";
+
+class _Destination {
+  final Widget icon;
+  final String label;
+
+  const _Destination({required this.icon, required this.label});
+}
+
+/// The app's only persistent navigation surface: a bottom [NavigationBar] on
+/// phone-width screens, a side [NavigationRail] on desktop-width ones — no
+/// hamburger drawer at either width. Mail/Contacts/Settings are switched by
+/// tapping only, never by swipe, so a horizontal gesture inside a
+/// destination (e.g. the Inbox/Sent/Requests tab bar) never changes section.
+@RoutePage()
+class HomeScreen extends ConsumerWidget {
+  const HomeScreen({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final unread = ref.watch(unreadProvider) + ref.watch(requestsUnreadProvider);
+    final appColors = Theme.of(context).extension<AppColors>()!;
+
+    final mailIcon = Icon(FlashMailIcons.inbox);
+    final destinations = [
+      _Destination(
+        icon: unread > 0 ? Badge(label: Text("$unread"), child: mailIcon) : mailIcon,
+        label: "Mail",
+      ),
+      const _Destination(icon: Icon(Icons.contacts), label: "Contacts"),
+      const _Destination(icon: Icon(Icons.settings), label: "Settings"),
+    ];
+
+    return AutoTabsRouter.builder(
+      routes: const [InboxRoute(), ContactsRoute(), SettingsRoute()],
+      builder: (context, children, tabsRouter) {
+        final body = IndexedStack(index: tabsRouter.activeIndex, children: children);
+        final wide = MediaQuery.sizeOf(context).width >= kDesktopBreakpoint;
+
+        if (!wide) {
+          return Scaffold(
+            body: body,
+            bottomNavigationBar: NavigationBar(
+              selectedIndex: tabsRouter.activeIndex,
+              onDestinationSelected: tabsRouter.setActiveIndex,
+              indicatorColor: appColors.highlight,
+              destinations: [
+                for (final d in destinations)
+                  NavigationDestination(icon: d.icon, label: d.label),
+              ],
+            ),
+          );
+        }
+
+        return Scaffold(
+          body: Row(
+            children: [
+              NavigationRail(
+                selectedIndex: tabsRouter.activeIndex,
+                onDestinationSelected: tabsRouter.setActiveIndex,
+                backgroundColor: appColors.cardFill,
+                useIndicator: true,
+                indicatorColor: appColors.highlight,
+                labelType: NavigationRailLabelType.all,
+                destinations: [
+                  for (final d in destinations)
+                    NavigationRailDestination(icon: d.icon, label: Text(d.label)),
+                ],
+              ),
+              const VerticalDivider(width: 1),
+              Expanded(child: body),
+            ],
+          ),
+        );
+      },
+    );
+  }
+}