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>

options/locale/locale_en-US.json

@@ -1043,6 +1043,7 @@
   "repo.forks": "Forks",
   "repo.stars": "Stars",
   "repo.reactions_more": "and %d more",
+  "repo.reactions": "Reactions",
   "repo.unit_disabled": "The site administrator has disabled this repository section.",
   "repo.language_other": "Other",
   "repo.adopt_search": "Enter username to search for unadopted repositories… (leave blank to find all)",