Fix Back button on SignIn page & Add Cancel button (#6415)

* Back button on SignIn page leads to Home page
Fixes #6413

* Add regression test for the bug, and for new cancel button

* Full sign in e2e test

* Good bot

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

---------

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

e2e/playwright/app-header-tests.spec.ts

@@ -29,11 +29,11 @@ test.describe('Electron app header tests', () => {
   test(
     'User settings has correct shortcut',
     { tag: '@electron' },
-    async ({ page }, testInfo) => {
+    async ({ page, toolbar }, testInfo) => {
       await page.setBodyDimensions({ width: 1200, height: 500 })
 
       // Open the user sidebar menu.
-      await page.getByTestId('user-sidebar-toggle').click()
+      await toolbar.userSidebarButton.click()
 
       // No space after "User settings" since it's textContent.
       const text =