fix(onboarding): give the focus ring back to the highlighted control

The ring of `[data-onboarding-highlight="true"]` is an author outline:
it wins over the browser's focus ring, so a keyboard user who reached
the highlighted control saw nothing change (WCAG 2.4.7, failure F78).
Measured by the review in headless Chromium, WebView2's engine: the
highlighted button kept `solid 3px` focused or not. `outline: revert`
under `:focus-visible` gives the browser's ring back, kept as is in the
built stylesheet; the contrast test reads the rule from the stylesheet.

Review of #398 (PR #404).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/styles.contrast.test.ts

@@ -4,7 +4,8 @@
 // - WCAG 1.4.11: the ring reaches 3:1 against the page background and the
 //   cards, in the light theme (`:root`) and the dark one (`.dark`);
 // - WCAG 2.2.2: the pulse stops by itself after three cycles, within 5 s;
-// - the pulse does not run under `prefers-reduced-motion: reduce`.
+// - the pulse does not run under `prefers-reduced-motion: reduce`;
+// - WCAG 2.4.7: the browser's focus ring replaces the ring on keyboard focus.
 import { describe, it, expect } from "vitest";
 import { readFileSync } from "fs";
 import { resolve } from "path";
@@ -161,4 +162,10 @@ describe("styles.css — the getting-started highlight (#398)", () => {
     expect(inner, "the highlight rule under reduced motion").toBeDefined();
     expect(declarations(inner!.body).get("animation")).toBe("none");
   });
+
+  it("gives the browser's focus ring back to the highlighted control (WCAG 2.4.7)", () => {
+    const rule = TOP.find((b) => b.selector === `${HIGHLIGHT}:focus-visible`);
+    expect(rule, "the focus-visible rule").toBeDefined();
+    expect(declarations(rule!.body).get("outline")).toBe("revert");
+  });
 });

src/styles.css

@@ -164,6 +164,16 @@ body {
   animation-iteration-count: 3;
 }
 
+/*
+ * An author outline wins over the browser's focus ring: without this rule, a
+ * keyboard user who reaches the highlighted control would see nothing change
+ * (WCAG 2.4.7). The browser's ring comes back while the control has the
+ * keyboard focus.
+ */
+[data-onboarding-highlight="true"]:focus-visible {
+  outline: revert;
+}
+
 @keyframes onboarding-pulse {
   from {
     box-shadow: 0 0 0 0 color-mix(in srgb, var(--accent) 55%, transparent);