Add e2e reaction test, improve accessibility, enable parallel testing (#37081)

Add a new e2e test for toggling issue reactions via the reaction picker
dropdown.

Add `aria-label` attributes to improve reaction accessibility:
- Add `aria-label="Reaction"` to the reaction picker dropdown
- Add `role="group"` with `aria-label="Reactions"` to the reactions
container, giving it a semantic identity for screen readers
- Include the reaction key in each reaction button's `aria-label` (e.g.
`+1: user1, user2`) so screen readers announce which reaction a button
represents

E2e test improvements:
- Simplify `randomString` to use `Math.random` instead of `node:crypto`
- Replace `generatePassword` with a static password, remove unused
`clickDropdownItem`
- Enable `fullyParallel: true` and `workers: '50%'` in Playwright config
- Run both chromium and firefox in all environments (not just CI)
- Parallelize `login` and `apiCreateRepo` setup where possible
- Use dedicated test user in `user-settings` test for concurrency safety

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

tests/e2e/org.test.ts

@@ -1,8 +1,8 @@
 import {test, expect} from '@playwright/test';
-import {login, apiDeleteOrg} from './utils.ts';
+import {login, apiDeleteOrg, randomString} from './utils.ts';
 
 test('create an organization', async ({page}) => {
-  const orgName = `e2e-org-${Date.now()}`;
+  const orgName = `e2e-org-${randomString(8)}`;
   await login(page);
   await page.goto('/org/create');
   await page.getByLabel('Organization Name').fill(orgName);