docs: add test conventions reference

[Copilot]

Captures ARIA and selector conventions from the login and dashboard pages into a single reference document so future Copilot agent sessions don't need to re-read source files to write correct selectors.

docs/test-conventions.md

• Login message roles — role="status" for success, role="alert" for errors; both carry aria-live="polite" (static). Specs target via getByRole('status') / getByRole('alert'), never #message.
• Login form selectors — getByRole('textbox', { name: 'Username' }), getByRole('textbox', { name: 'Password' }), getByRole('button', { name: 'Login' }).
• Stat card structure — Each card has an <h3> label + <p class="stat" id="…"> value. IDs: #user-count, #revenue, #order-count. Tests use getByRole('heading', { name, level: 3 }) for labels, locator('#id') for values.
• Chart-skeleton loading — div.chart-skeleton with role="status" and aria-label="Loading {label} chart" for the first 900 ms. Parent grid carries aria-busy={isChartsLoading}.
• prefers-reduced-motion — CSS-only (animation: none + static background inside the media query). No JS hook; use page.emulateMedia({ reducedMotion: 'reduce' }) in Playwright tests.

Five corresponding Copilot Memory entries were stored so heal/review agents can use these conventions without additional file reads.

Original prompt

Please read these source and test files and produce a short, factual conventions document for future Copilot sessions in this repo:

• src/pages/Login.js
• tests/login.spec.ts
• src/pages/Dashboard.js (or wherever stat cards live)
• tests/dashboard.spec.ts

Document the following with concrete details (actual role names, selectors, file paths):

1. Which ARIA roles the login message uses for success vs error (and aria-live value).
2. How Playwright specs assert on those messages — selector strategy (getByRole, getByLabel, etc.).
3. The dashboard stat cards' structure and how tests target them.
4. The chart-skeleton loading state pattern (role, aria-label).
5. Any prefers-reduced-motion handling.

Important: please remember these conventions in this repository's Copilot Memory so future agent sessions (heal, code review, etc.) can use them without re-reading the source. Be concrete: include the actual role names, selectors, and file paths.

Output: open a small PR that adds a docs/test-conventions.md file with these facts. Do not modify production code or tests. The PR title should be exactly: "docs: add test conventions reference".

This is part of a proof-of-concept measuring whether GitHub Copilot Memory captures heal-relevant facts (selector and ARIA conventions) when an agent is explicitly asked to remember them. The PR diff itself is secondary; the primary goal is the Memory entries produced by this session.