Improve async validation UX in the support form

[Abidoyesimze]

Summary

Support form async validation lacks clear loading, cancellation, and field-level feedback.

Affected files

• src/components/help/SupportForm.tsx
• src/lib/form-validation.ts

Tasks

• Show per-field loading during async validation.
• Cancel in-flight validation when input changes.
• Surface clearer inline error messages.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

cleanup, a11y, ux

Notes

Standalone cleanup issue from CLEANUP_ISSUES_2026-05.md. Complements the audit queue and product backlog.