feat(auth): build sign-in form and validation states

[ayushtr-aws]

Summary

Implements TG-17: Build sign-in form and validation states — an accessible sign-in form that consumes the shared authentication state model added in the parent epic.

What was added

• src/components/SignInForm.tsx — a controlled, accessible sign-in form:
  • Email and password inputs with associated <label htmlFor> bindings, autoComplete, and appropriate type/inputMode.
  • Client-side validation reusing the shared isValidEmail/isValidPassword helpers from src/auth. Fields report required/format/length errors; invalid fields set aria-invalid and link their message via aria-describedby. Errors re-validate live after the first submit attempt so users get feedback as they correct input.
  • Loading feedback — while onSubmit is pending (or the external loading prop is set) the button shows "Signing in…", sets aria-busy, and inputs are disabled. Supports both internally-managed (promise-based) and externally-controlled loading.
  • Error feedback — form-level errors render in a role="alert" region, sourced from either a rejected onSubmit promise or an externally-provided error prop (e.g. an expired auth state).
  • Keyboard accessibility — native form semantics; Tab order flows email → password → submit, and Enter submits.
• src/components/SignInForm.css — styling consistent with the app's existing dark theme, including visible focus rings and invalid-field states.
• src/components/SignInForm.test.tsx — 11 component tests covering rendering, empty/format/length validation, aria wiring, successful submit, live error clearing, internal + external loading states, rejection and external error display, and keyboard operation.

Test infrastructure

The repo had Vitest configured but no DOM environment or React testing utilities. Added:

• Dev deps: @testing-library/react, @testing-library/user-event, @testing-library/jest-dom, jsdom.
• src/test/setup.ts — registers jest-dom matchers and cleans up between tests.
• vitest.config.ts — a separate config (jsdom environment + setup file). It is kept apart from vite.config.ts because the repo uses Vite 8 (rolldown) at the root while Vitest bundles Vite 7; combining the two plugin types in one config trips tsc -b. The test npm script points at this config.

Build & test results

• mise run build (tsc -b && vite build): PASS — built in ~150ms, no type errors.
• mise run lint (oxlint): PASS — no warnings.
• npm test (vitest run): PASS — 33/33 tests (11 new component tests + 22 existing auth tests).

Decisions made

• Reused the shared model's isValidEmail/isValidPassword rather than re-implementing validation, keeping this UI aligned with sibling issues.
• Exposed both a promise-driven flow and optional error/loading props so the form works standalone or wired to externally-managed auth state.
• The form only collects and validates credentials and invokes onSubmit; it deliberately does not implement the authentication request or global state wiring, which belong to sibling issues in the epic.

Agent notes

• What went well: The shared src/auth module was clean and pure, so validation reuse was straightforward. Native form semantics gave keyboard accessibility for free.
• What was difficult: Getting Vitest to type-check and build cleanly. A Vite 7 (bundled with Vitest) vs Vite 8 (rolldown, project root) plugin-type mismatch made adding test to vite.config.ts fail tsc -b; isolating it in vitest.config.ts resolved it without touching the production build config.
• Conventions discovered: Conventional-commit messages with a module scope; TypeScript is strict (noUnusedLocals/noUnusedParameters, verbatimModuleSyntax requiring import type); oxlint with react/typescript plugins; components live in src/components and are function components with named exports.
• Suggestions for future tasks: Consider aligning the root and Vitest Vite versions to allow a single config. A shared AuthProvider/context would let this form wire directly into global auth state.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.