feat: add sliding animation on tab change

lib/router.dart

@@ -1,6 +1,8 @@
+import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import 'features/connection/startup_screen.dart';
 import 'features/connection/server_discovery_screen.dart';
 import 'features/connection/login_screen.dart';
 import 'features/onboarding/onboarding_screen.dart';
@@ -13,20 +15,21 @@ import 'features/tags/tag_detail_screen.dart';
 import 'features/settings/settings_screen.dart';
 import 'providers.dart';
 
-const _unauthenticated = {'/connect', '/login', '/onboarding'};
+// Routes that manage their own auth state — no redirect applied.
+const _unauthenticated = {'/', '/connect', '/login', '/onboarding'};
 
 final routerProvider = Provider<GoRouter>((ref) {
   return GoRouter(
-    initialLocation: '/connect',
+    initialLocation: '/',
     redirect: (context, state) {
+      final loc = state.matchedLocation;
+      if (_unauthenticated.contains(loc)) return null;
+
       final hasConfig = ref.read(serverConfigProvider) != null;
       final hasToken = ref.read(authTokenProvider) != null;
-      final loc = state.matchedLocation;
 
-      if (!hasConfig && loc != '/connect') return '/connect';
-      if (hasConfig && !hasToken && !_unauthenticated.contains(loc)) {
-        return '/login';
-      }
+      if (!hasConfig) return '/connect';
+      if (!hasToken) return '/login';
       return null;
     },
     routes: [
@@ -46,8 +49,10 @@ final routerProvider = Provider<GoRouter>((ref) {
         path: '/onboarding',
         builder: (context, state) => const OnboardingScreen(),
       ),
-      StatefulShellRoute.indexedStack(
+      StatefulShellRoute(
         builder: (context, state, shell) => MainShell(shell: shell),
+        navigatorContainerBuilder: (context, shell, children) =>
+            _SlidingTabSwitcher(index: shell.currentIndex, children: children),
         branches: [
           StatefulShellBranch(routes: [
             GoRoute(
@@ -94,3 +99,87 @@ final routerProvider = Provider<GoRouter>((ref) {
     ],
   );
 });
+
+class _SlidingTabSwitcher extends StatefulWidget {
+  const _SlidingTabSwitcher({required this.index, required this.children});
+
+  final int index;
+  final List<Widget> children;
+
+  @override
+  State<_SlidingTabSwitcher> createState() => _SlidingTabSwitcherState();
+}
+
+class _SlidingTabSwitcherState extends State<_SlidingTabSwitcher>
+    with SingleTickerProviderStateMixin {
+  late AnimationController _controller;
+  late Animation<double> _animation;
+  late int _current;
+  int _previous = -1;
+  bool _forward = true;
+
+  @override
+  void initState() {
+    super.initState();
+    _current = widget.index;
+    _controller = AnimationController(
+      vsync: this,
+      duration: const Duration(milliseconds: 220),
+      value: 1.0,
+    );
+    _animation = CurvedAnimation(parent: _controller, curve: Curves.easeInOut);
+  }
+
+  @override
+  void didUpdateWidget(_SlidingTabSwitcher old) {
+    super.didUpdateWidget(old);
+    if (widget.index != _current) {
+      _previous = _current;
+      _forward = widget.index > _current;
+      setState(() => _current = widget.index);
+      _controller.forward(from: 0);
+    }
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return AnimatedBuilder(
+      animation: _animation,
+      builder: (context, _) {
+        return Stack(
+          fit: StackFit.expand,
+          children: List.generate(widget.children.length, (i) {
+            if (i == _current) {
+              final begin = Offset(_forward ? 1.0 : -1.0, 0);
+              final offset = Offset.lerp(begin, Offset.zero, _animation.value)!;
+              return FractionalTranslation(
+                translation: offset,
+                child: TickerMode(enabled: true, child: widget.children[i]),
+              );
+            }
+            if (i == _previous) {
+              final end = Offset(_forward ? -1.0 : 1.0, 0);
+              final offset = Offset.lerp(Offset.zero, end, _animation.value)!;
+              return FractionalTranslation(
+                translation: offset,
+                child: IgnorePointer(
+                  child: TickerMode(enabled: false, child: widget.children[i]),
+                ),
+              );
+            }
+            return Offstage(
+              offstage: true,
+              child: TickerMode(enabled: false, child: widget.children[i]),
+            );
+          }),
+        );
+      },
+    );
+  }
+}