docs(readme): align quick contribution guide with Conventional Commit…

[xeladev4]

fix(frontend, docs): accessible LanguageSwitcher, AbortSignal forwarding, ENVIRONMENT.md source fixes, and useTransaction selector perf

Summary

This PR bundles four related frontend/docs hardening fixes that came out of the same accessibility/performance/documentation sweep:

1. LanguageSwitcher accessible name (Closes #1229) — the language select now exposes a programmatic accessible name (aria-label="Language"), and the decorative Languages icon is marked aria-hidden so screen readers no longer announce an unlabeled combobox (WCAG 4.1.2 / 1.3.1).
2. apiFetch AbortSignal forwarding (Closes #1221) — apiFetch now accepts an optional signal?: AbortSignal that is passed into fetch(), and query functions forward TanStack Query's context.signal. Navigating away / refetching and queryClient.cancelQueries in optimistic mutations now actually abort in-flight requests.
3. ENVIRONMENT.md Source column accuracy (Closes #1210) — corrected the Source column for backend variables to point at the real files (config/loanConfig.ts, config/stellar.ts, config/env.ts, config/sentry.ts, …) instead of non-existent paths.
4. useTransaction store selector (Closes #1223) — useTransaction now selects only transactions[id] plus the specific stable action references it needs, instead of subscribing to the whole OptimisticUI store, eliminating re-renders of every consumer on unrelated transaction updates and dropping store from the auto-dismiss effect deps.

Also tidied the README Quick Contribution Guide so its branch-name and commit examples match the Conventional Commits + branch-naming conventions in CONTRIBUTING.md.

Linked Issues

Closes #1229
Closes #1221
Closes #1210
Closes #1223

Files Touched

• frontend/src/app/components/global_ui/LanguageSwitcher.tsx
• frontend/src/app/hooks/useApi.ts
• docs/ENVIRONMENT.md
• frontend/src/app/hooks/useOptimisticUI.ts
• README.md

Testing

• frontend: npm run lint and npm run test pass; added a test querying the language control by accessible role/name to verify the accessible name.
• Verified an unmounted / navigated-away query aborts its request in the browser devtools Network panel.
• Confirmed each corrected Source path in docs/ENVIRONMENT.md resolves to an existing file.

Checklist

• Code follows project style guides.
• Tests have been added/updated and pass.
• Documentation has been updated.
• Commit messages follow standards.

Out of Scope

• Adding new locales or wiring full i18n persistence ([Frontend] LanguageSwitcher select has no accessible name (no label or aria-label) #1229).
• SSE stream cancellation, already handled via AbortController ([Frontend] apiFetch ignores TanStack Query's AbortSignal, so query cancellation and onMutate cancelQueries never abort in-flight requests #1221).
• Frontend/contracts Source columns in ENVIRONMENT.md unless similarly wrong ([Docs] ENVIRONMENT.md 'Source' column points to many non-existent files #1210).
• Transaction state-machine semantics ([Frontend] useTransaction subscribes to the entire OptimisticUI store with no selector, re-rendering every consumer on any transaction update #1223).