Feature: Add Settings module with PanelNavigator, AppSettingsView, and related components for nested navigation + theming

finlog_app/app/lib/core/ui/panel.dart

@@ -0,0 +1,183 @@
+import 'package:flutter/material.dart';
+import 'package:animations/animations.dart';
+
+/// Ermöglicht verschachtelte "Panels" ähnlich einem StackNavigator.
+/// Beispiel:
+/// ```dart
+/// PanelNavigator(
+///   rootBuilder: (_) => MyRootPanel(),
+/// )
+/// ```
+class PanelNavigator extends StatefulWidget {
+  final WidgetBuilder rootBuilder;
+
+  const PanelNavigator({super.key, required this.rootBuilder});
+
+  @override
+  State<PanelNavigator> createState() => _PanelNavigatorState();
+
+  /// Zugriff per InheritedWidget / Extension
+  static PanelController of(BuildContext context) {
+    final inherited = context
+        .dependOnInheritedWidgetOfExactType<_PanelInherited>();
+    assert(inherited != null, 'Kein PanelNavigator im Widget-Tree gefunden');
+    return inherited!.controller;
+  }
+}
+
+class _PanelNavigatorState extends State<PanelNavigator> {
+  late final PanelController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = PanelController(
+      rootBuilder: widget.rootBuilder,
+      onUpdate: () => setState(() {}),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return _PanelInherited(
+      controller: _controller,
+      child: PageTransitionSwitcher(
+        duration: const Duration(milliseconds: 300),
+        reverse: _controller.navDirection == _NavDirection.back,
+        transitionBuilder: (child, primary, secondary) {
+          return SharedAxisTransition(
+            animation: primary,
+            secondaryAnimation: secondary,
+            transitionType: SharedAxisTransitionType.horizontal,
+            fillColor: Colors.transparent,
+            child: child,
+          );
+        },
+        child: _controller.getCurrentPanel(context),
+      ),
+    );
+  }
+}
+
+class PanelController {
+  final WidgetBuilder rootBuilder;
+  final VoidCallback onUpdate;
+
+  final List<_PanelEntry> _stack = [];
+  _NavDirection _navDirection = _NavDirection.forward;
+  int _version = 0;
+
+  PanelController({required this.rootBuilder, required this.onUpdate}) {
+    _stack.add(_PanelEntry(title: null, builder: rootBuilder));
+  }
+
+  _NavDirection get navDirection => _navDirection;
+
+  Widget getCurrentPanel(BuildContext context) {
+    final entry = _stack.last;
+    return KeyedSubtree(
+      key: ValueKey('panel_v${_version}_${_stack.length}'),
+      child: _PanelScaffold(
+        title: entry.title,
+        body: entry.builder(context),
+        onBack: canPop ? pop : null,
+      ),
+    );
+  }
+
+  bool get canPop => _stack.length > 1;
+
+  void push(WidgetBuilder builder, {String? title}) {
+    _navDirection = _NavDirection.forward;
+    _stack.add(_PanelEntry(title: title, builder: builder));
+    _version++;
+    onUpdate();
+  }
+
+  void pop() {
+    if (canPop) {
+      _navDirection = _NavDirection.back;
+      _stack.removeLast();
+      _version++;
+      onUpdate();
+    }
+  }
+}
+
+enum _NavDirection { forward, back }
+
+class _PanelEntry {
+  final String? title;
+  final WidgetBuilder builder;
+
+  _PanelEntry({this.title, required this.builder});
+}
+
+class _PanelScaffold extends StatelessWidget {
+  final String? title;
+  final Widget body;
+  final VoidCallback? onBack;
+
+  const _PanelScaffold({required this.title, required this.body, this.onBack});
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: [
+        if (title != null) PanelHeader(title: title!, onBack: onBack),
+        Expanded(
+          child: Padding(padding: const EdgeInsets.all(15), child: body),
+        ),
+      ],
+    );
+  }
+}
+
+class _PanelInherited extends InheritedWidget {
+  final PanelController controller;
+
+  const _PanelInherited({required super.child, required this.controller});
+
+  @override
+  bool updateShouldNotify(_PanelInherited oldWidget) =>
+      controller != oldWidget.controller;
+}
+
+/// ---------- PUBLIC UI COMPONENTS ----------
+
+class PanelHeader extends StatelessWidget {
+  final String title;
+  final VoidCallback? onBack;
+
+  const PanelHeader({super.key, required this.title, this.onBack});
+
+  @override
+  Widget build(BuildContext context) {
+    final scheme = Theme.of(context).colorScheme;
+    return Material(
+      color: scheme.surface,
+      elevation: 1,
+      child: SizedBox(
+        height: 56,
+        child: Row(
+          children: [
+            if (onBack != null)
+              IconButton(icon: const Icon(Icons.arrow_back), onPressed: onBack),
+            Expanded(
+              child: Text(
+                title,
+                style: Theme.of(context).textTheme.titleMedium,
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+/// ---------- EXTENSIONS ----------
+
+extension PanelContext on BuildContext {
+  PanelController get panels => PanelNavigator.of(this);
+}