FIX: add a skip-to-main-content link for keyboard users

[Akshar Gothi (akshar27)]

Description

The application shell has no skip link or equivalent bypass control. On every route, keyboard users must Tab through the same shell controls (Take a tour, primary navigation, Feedback, Security, Theme) before reaching page content — 10 Tab presses on the Home page per the issue. This doesn't satisfy the bypass-blocks expectation in WCAG 2.4.1.

Adds a visually-hidden-until-focused skip link as the very first focusable element in MainLayout, targeting the existing main landmark referenced in the issue (MainLayout.tsx:89). Gave <main> a stable id="main-content" and tabIndex={-1} so activating the link moves keyboard focus directly into the landmark (not just the scroll position, which a plain anchor jump alone wouldn't reliably do for a non-naturally-focusable element).

The skip link reuses the existing Fluent UI brand tokens (colorBrandBackground/colorNeutralForegroundOnBrand) already used elsewhere in the app for prominent elements, so it's visually consistent with the rest of the design system when it appears on focus.

Fixes #2597

Tests and Documentation

• Added a new test to MainLayout.test.tsx asserting: the skip link renders with href="#main-content", <main> has the matching id and tabIndex={-1}, and the skip link is the first focusable element among a[href]/button/[tabindex] in the rendered tree (this is what guarantees it's reached on the very first Tab press).
• npm test — all 1409 tests pass (70 suites).
• npm run lint / npm run type-check / npm run build — all clean.
• Manually verified end-to-end in a running dev server (npm run dev, with a minimal stub for /api/auth/config returning empty client/tenant IDs to exercise the app's existing no-auth dev path): confirmed via the DOM that (1) the skip link is the first focusable element within the app root, (2) it's positioned off-screen by default (top: -40px) and only reveals itself when it receives keyboard-driven focus (Griffel's generated :focus-visible rule sets top: 0), and (3) activating it moves document.activeElement to the <main> landmark (id="main-content"), not just the scroll position. I couldn't get the CDP-synthesized Tab key to trigger Chromium's native focus traversal reliably in the browser-automation environment I have access to, so I verified the same underlying behavior directly instead — happy for a reviewer to also spot-check with a real Tab press.

Notes

• GitHub Actions doesn't appear to be enabled yet on my fork (akshar27/PyRIT) — a one-time manual step outside this PR. I'll push a retrigger commit once it's on, if CI doesn't pick this PR up.

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[Akshar Gothi (akshar27)]

Both addressed in c77392b:

1. Switched to transform: translateY(-100%) (with top: 0 as the anchor) instead of the fixed top: -40px, so the link stays fully off-screen regardless of its own rendered height. Also added transitionDuration: '0s' under @media (prefers-reduced-motion: reduce) so it snaps instead of sliding for users who request that.
2. Added a real Playwright e2e test in e2e/accessibility.spec.ts that presses Tab, asserts the link is focused and visible, presses Enter, and asserts #main-content receives focus. Since the shared beforeEach in that file depends on a real backend I don't have running for this test's own boot sequence, I mocked the boot-time /api/* calls the same way labels-operation-picker.spec.ts already does, so the test is self-contained and doesn't depend on backend availability. Ran it locally against a real Chromium — passes.

[Roman Lutz (romanlutz)]

You'll need to accept the CLA otherwise we can't accept the contribution. Akshar Gothi (@akshar27)