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/capabilities/default.json

@@ -19,6 +19,8 @@
     {
       "identifier": "fs:allow-read-file",
       "allow": [{ "path": "**" }]
-    }
+    },
+    "opener:default",
+    "shell:allow-open"
   ]
 }