Add /register web route to serve SPA for invitation registration

The SPA catch-all only handles /admin/{vue?}. The /register route needs
its own web route to serve the app view so Vue router can handle it.

routes/web.php

@@ -123,6 +123,13 @@ Route::get('/installation', function () {
 })->name('install')
     ->middleware(['redirect-if-installed']);
 
+// Registration via invitation (serves SPA)
+// -------------------------------------------------
+
+Route::get('/register', function () {
+    return view('app');
+})->middleware(['install']);
+
 // Move other http requests to the Vue App
 // -------------------------------------------------