feat: show back and forward swipes as visible page transitions

app/lib/application/controllers/browser_controller.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:actors/actors.dart';
 import 'package:drift/drift.dart' show Value;
 import 'package:signals/signals.dart';
@@ -45,6 +47,10 @@ class BrowserController {
   /// Pages opened this session, so links can show the visited color.
   final visitedUrls = signal(<String>{});
 
+  /// Titles of the stack's neighboring pages, shown while swiping.
+  final backTitle = signal<String?>(null);
+  final forwardTitle = signal<String?>(null);
+
   bool get canGoBack => _stackIndex > 0;
   bool get canGoForward => _stackIndex >= 0 && _stack.length - 1 > _stackIndex;
 
@@ -63,18 +69,21 @@ class BrowserController {
     _stack.removeRange(_stackIndex + 1, _stack.length);
     _stack.add(url);
     _stackIndex++;
+    _updatePeeks();
     await _loadCurrent();
   }
 
   Future<void> back() async {
     if (!canGoBack) return;
     _stackIndex--;
+    _updatePeeks();
     await _loadCurrent();
   }
 
   Future<void> forward() async {
     if (!canGoForward) return;
     _stackIndex++;
+    _updatePeeks();
     await _loadCurrent();
   }
 
@@ -90,6 +99,7 @@ class BrowserController {
         _stack.removeRange(_stackIndex + 1, _stack.length);
         _stack.add(page.url);
         _stackIndex++;
+        _updatePeeks();
         _show(page);
       case _:
         error.value = const LoadFailed();
@@ -102,9 +112,23 @@ class BrowserController {
     if (url == null) return;
     _stack.add(url);
     _stackIndex = 0;
+    _updatePeeks();
     await _loadCurrent();
   }
 
+  /// Refreshes the neighboring-page titles from the cache; a page without a
+  /// cached title falls back to its URL.
+  void _updatePeeks() {
+    Future<String?> peek(int index) async {
+      if (index < 0 || index >= _stack.length) return null;
+      final url = _stack[index];
+      return (await _db.cachedPage(url))?.title ?? url;
+    }
+
+    unawaited(peek(_stackIndex - 1).then((t) => backTitle.value = t));
+    unawaited(peek(_stackIndex + 1).then((t) => forwardTitle.value = t));
+  }
+
   Future<void> _loadCurrent() async {
     if (_stackIndex < 0) return;
     final url = _stack[_stackIndex];