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/repo-code.test.e2e.ts

@@ -5,11 +5,7 @@
 // @watch end
 
 import {expect} from '@playwright/test';
-import {test, login_user, load_logged_in_context} from './utils_e2e.ts';
-
-test.beforeAll(async ({browser}, workerInfo) => {
-  await login_user(browser, workerInfo, 'user2');
-});
+import {test} from './utils_e2e.ts';
 
 async function assertSelectedLines(page, nums) {
   const pageAssertions = async () => {
@@ -33,10 +29,7 @@ async function assertSelectedLines(page, nums) {
   return pageAssertions();
 }
 
-test('Line Range Selection', async ({browser}, workerInfo) => {
-  const context = await load_logged_in_context(browser, workerInfo, 'user2');
-  const page = await context.newPage();
-
+test('Line Range Selection', async ({page}) => {
   const filePath = '/user2/repo1/src/branch/master/README.md?display=source';
 
   const response = await page.goto(filePath);