tests(e2e): Refactor various tests

Goals:

- speedup
- less flakiness
- best practices and more use
- documentation

config:
- sync ports in Makefile and playwright config
  (otherwise, some tests fail locally because they assert the full URL including the (wrong) port)
- even more generous timeouts
- limit workers to one again (because I finally understand how
  Playwright works)
- allow nested functions to group them together with the related test

all:

- deprecate waitForLoadState('networkidle')
  - it is discouraged as per https://playwright.dev/docs/api/class-page#page-wait-for-load-state
  - I could not find a usage that seems to require it actually (see
    added documentation in README)
  - adding an exception should be made explicitly
  - it does not do what you might expect anyway in most cases
- only log in when necessary

webauthn:

- verify that login is possible after disabling key
- otherwise, the cleanup was not necessary after the previous refactor to create a fresh user each

issue-sidebar / WIP toggle:

- split into smaller chunks
- restore original state first
- add missed assertion to fix race condition (not waiting
  before state was reached)
- explicitly toggle the state to detect mismatch earlier

issue-sidebar / labels:

- restore original state first
- better waiting for background request

tests/e2e/issue-comment.test.e2e.ts

@@ -56,7 +56,7 @@ test('Always focus edit tab first on edit', async ({browser}, workerInfo) => {
   await page.locator('#issue-1 .comment-container a[data-tab-for=markdown-previewer]').click();
   await page.click('#issue-1 .comment-container .save');
 
-  await page.waitForLoadState('networkidle');
+  await page.waitForLoadState();
 
   // Edit again and assert that edit tab should be active (and not preview tab)
   await page.click('#issue-1 .comment-container .context-menu');