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>

Makefile

@@ -525,7 +525,7 @@ test-mssql-migration: migrations.mssql.test migrations.individual.mssql.test
 .PHONY: playwright
 playwright: deps-frontend
 	@# on GitHub Actions VMs, playwright's system deps are pre-installed
-	@pnpm exec playwright install $(if $(GITHUB_ACTIONS),,--with-deps) chromium $(if $(CI),firefox) $(PLAYWRIGHT_FLAGS)
+	@pnpm exec playwright install $(if $(GITHUB_ACTIONS),,--with-deps) chromium firefox $(PLAYWRIGHT_FLAGS)
 
 .PHONY: test-e2e
 test-e2e: playwright $(EXECUTABLE_E2E)