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>

templates/repo/issue/view_content/add_reaction.tmpl

@@ -1,5 +1,5 @@
 {{if ctx.RootData.IsSigned}}
-<div class="item action ui dropdown jump pointing top right select-reaction" data-action-url="{{.ActionURL}}">
+<div class="item action ui dropdown jump pointing top right select-reaction" data-action-url="{{.ActionURL}}" aria-label="{{ctx.Locale.Tr "repo.reactions"}}">
 	<a class="muted">{{svg "octicon-smiley"}}</a>
 	<div class="menu">
 		{{range $value := AllowedReactions}}