Add Sign Up functionality to desktop app (#5244)

# Description of Changes
Adds Sign Up with email to desktop app. SSO sign up will come in a
future PR.

frontend/src-tauri/tauri.conf.json

@@ -77,6 +77,13 @@
         },
         "fs": {
             "requireLiteralLeadingDot": false
+        },
+        "deep-link": {
+            "desktop": {
+                "schemes": [
+                    "stirlingpdf"
+                ]
+            }
         }
     }
 }