Extract a per-user background host so indexing can later run outside the window.

Keep the GUI as the index writer for now; mutex, named pipe, and opt-in logon autostart prepare Explorer.Host.exe without two SQLite writers.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/Explorer.Application.Tests/UiPreferencesStoreTests.cs

@@ -43,6 +43,18 @@ public class UiPreferencesStoreTests
         Assert.Equal(@"C:\Program Files\Git\cmd\git.exe", prefs.GitPath);
     }
 
+    [Fact]
+    public void Parse_reads_host_and_removable_index_flags()
+    {
+        var prefs = UiPreferencesStore.Parse(
+        [
+            "auto-index-removable=true",
+            "background-host-at-logon=true"
+        ]);
+        Assert.True(prefs.AutoIndexRemovable);
+        Assert.True(prefs.BackgroundHostAtLogon);
+    }
+
     [Fact]
     public void Parse_defaults_missing_keys()
     {
@@ -54,6 +66,8 @@ public class UiPreferencesStoreTests
         Assert.True(prefs.ShowHiddenFiles);
         Assert.False(prefs.ShowProtectedSystemLocations);
         Assert.False(prefs.AutoClearQueueWhenDone);
+        Assert.False(prefs.AutoIndexRemovable);
+        Assert.False(prefs.BackgroundHostAtLogon);
         Assert.Null(prefs.SevenZipPath);
         Assert.Null(prefs.GitPath);
     }
@@ -95,6 +109,8 @@ public class UiPreferencesStoreTests
             Assert.True(loaded.ShowHiddenFiles);
             Assert.False(loaded.ShowProtectedSystemLocations);
             Assert.True(loaded.AutoClearQueueWhenDone);
+            Assert.False(loaded.AutoIndexRemovable);
+            Assert.False(loaded.BackgroundHostAtLogon);
             Assert.Equal(1100, loaded.WindowWidth);
             Assert.Equal(720, loaded.WindowHeight);
             Assert.Equal(300, loaded.TreeWidth);