feat(avalonia): implement HighlightTextConverter with theme-aware styling

- Add gold foreground color (#FFD700) for highlighted search matches
- Update HighlightTextConverter to use theme resource with fallback
- Fix TextBlockHelper to preserve Foreground property when copying Runs
- Fix highlight update bug: sync TitleHighlightData/SubTitleHighlightData/Score
  for reused ResultViewModel instances when query changes
- Update migration checklist to mark HighlightTextConverter as done

AVALONIA_MIGRATION_CHECKLIST.md

@@ -281,7 +281,7 @@
 
 | Converter | Status | Description |
 |-----------|--------|-------------|
-| HighlightTextConverter | :yellow_circle: Stub | Returns plain text, needs highlighting |
+| HighlightTextConverter | :white_check_mark: Done | Bold gold highlighting for matched characters |
 | QuerySuggestionBoxConverter | :yellow_circle: Stub | Not implemented |
 | BoolToVisibilityConverter | :white_check_mark: Done | Boolean to visibility |
 | TextConverter | :x: Missing | Text transformations |

Flow.Launcher.Avalonia/Converters/CommonConverters.cs

@@ -57,7 +57,18 @@ public class HighlightTextConverter : IMultiValueConverter
     {
         var run = new Run(text);
         if (isHighlight)
+        {
             run.FontWeight = FontWeight.Bold;
+            // Try to get from resources, fallback to gold
+            if (Application.Current != null && Application.Current.TryGetResource("HighlightForegroundBrush", null, out var brush) && brush is IBrush b)
+            {
+                run.Foreground = b;
+            }
+            else
+            {
+                run.Foreground = new SolidColorBrush(Colors.Gold);
+            }
+        }
         return run;
     }
 }

Flow.Launcher.Avalonia/Helper/TextBlockHelper.cs

@@ -42,7 +42,8 @@ public static class TextBlockHelper
                 {
                     var newRun = new Run(run.Text)
                     {
-                        FontWeight = run.FontWeight
+                        FontWeight = run.FontWeight,
+                        Foreground = run.Foreground
                     };
                     textBlock.Inlines?.Add(newRun);
                 }

Flow.Launcher.Avalonia/Themes/Resources.axaml

@@ -11,6 +11,7 @@
     <Color x:Key="ResultSubTitleColor">#999999</Color>
     <Color x:Key="SeparatorColor">#333333</Color>
     <Color x:Key="SelectedItemBackgroundColor">#30FFFFFF</Color>
+    <Color x:Key="HighlightForegroundColor">#FFD700</Color>
     
     <!-- Brushes -->
     <SolidColorBrush x:Key="WindowBackgroundBrush" Color="{StaticResource WindowBackgroundColor}" />
@@ -19,5 +20,6 @@
     <SolidColorBrush x:Key="ResultSubTitleBrush" Color="{StaticResource ResultSubTitleColor}" />
     <SolidColorBrush x:Key="SeparatorBrush" Color="{StaticResource SeparatorColor}" />
     <SolidColorBrush x:Key="SelectedItemBackgroundBrush" Color="{StaticResource SelectedItemBackgroundColor}" />
+    <SolidColorBrush x:Key="HighlightForegroundBrush" Color="{StaticResource HighlightForegroundColor}" />
 
 </ResourceDictionary>

Flow.Launcher.Avalonia/ViewModel/ResultsViewModel.cs

@@ -62,6 +62,21 @@ public partial class ResultsViewModel : ObservableObject, IDisposable
             r.Settings = _settings;
         }
 
+        // Update highlight data and score for items that will be kept by EditDiff.
+        // This is necessary because EditDiff reuses existing ResultViewModel instances
+        // based on Title+SubTitle equality, but highlight indices are query-specific.
+        // For example, "Chrome" highlighted for "chr" [0,1,2] vs "chrome" [0,1,2,3,4,5].
+        var existingItems = _sourceList.Items.ToDictionary(r => (r.Title, r.SubTitle));
+        foreach (var newItem in resultsList)
+        {
+            if (existingItems.TryGetValue((newItem.Title, newItem.SubTitle), out var existing))
+            {
+                existing.TitleHighlightData = newItem.TitleHighlightData;
+                existing.SubTitleHighlightData = newItem.SubTitleHighlightData;
+                existing.Score = newItem.Score;
+            }
+        }
+
         // EditDiff calculates minimal changes needed - items with same Title+SubTitle are kept
         _sourceList.EditDiff(resultsList, ResultViewModelComparer.Instance);