Register the tray's real path at startup

src/DiffEngineTray/Startup.cs

@@ -2,12 +2,32 @@
 
 public class Startup
 {
+    /// <summary>
+    /// Registers this executable, wherever it actually is.
+    /// <para>
+    /// The path used to be guessed as <c>%USERPROFILE%\.dotnet\tools\DiffEngineTray.exe</c>, which
+    /// is only right for a default global install. A <c>--tool-path</c> install, a
+    /// <c>DOTNET_CLI_HOME</c> that moves the tools directory, or simply running a local build all
+    /// registered a path with nothing at it - so the tray never started at login, while the
+    /// Options checkbox, which reads the settings file rather than the registry, went on reporting
+    /// that it would.
+    /// </para>
+    /// <para>
+    /// Quoted, because a Run value is a command line: an unquoted path containing a space is read
+    /// as a program name followed by arguments.
+    /// </para>
+    /// </summary>
     public static void Add()
     {
-        var profile = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
-        var exePath = Path.Combine(profile, ".dotnet", "tools", "DiffEngineTray.exe");
+        var exePath = Environment.ProcessPath;
+        if (exePath == null)
+        {
+            // No host path to register, which a normal launch does not produce
+            return;
+        }
+
         using var key = GetRunKey();
-        key.SetValue("DiffEngineTray", exePath);
+        key.SetValue("DiffEngineTray", $"\"{exePath}\"");
     }
 
     public static void Remove()
@@ -25,4 +45,4 @@ public static bool Exists()
     static RegistryKey GetRunKey() =>
         Registry.CurrentUser
             .OpenSubKey(@"SOFTWARE\Microsoft\Windows\CurrentVersion\Run", true)!;
-}
+}