feat(theme): add semantic tokens, focus ring, and reduced-motion support

Adds the layer feature code has been missing: tokens that name a role — page
surface, subtle border, danger — instead of a colour. Every one resolves to a
Mantine variable rather than a literal, so they follow the colour scheme for
free and cannot drift from the theme. A parallel palette of raw hexes would
have recreated exactly the problem this exists to fix.

Also closes three accessibility gaps that had no implementation anywhere in the
codebase: no :focus-visible rule, no screen-reader-only utility, and no global
prefers-reduced-motion handling.

The focus work found a real bug, and nearly introduced a worse one. Mantine
already rings its own controls, so the first attempt deferred to it with
`outline: none` on .mantine-focus-auto. That suppressed Mantine's ring without
replacing it, leaving portal buttons with no focus indicator at all — and it
only showed up in one app, because which rule won depended on stylesheet order.
The rules use identical values, so overlapping them is invisible and safe;
opting out is not. The focus test now asserts computed outline width and style,
not just pixels, since a screenshot alone would not have caught this.

`status-tone.ts` establishes the six-tone vocabulary that the 48 scattered
status→colour maps will eventually collapse onto. Nothing consumes it yet.

No visual change: the 8 existing baselines pass unmodified.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/e2e/visual/theme.spec.ts

@@ -58,4 +58,43 @@ for (const app of APPS) {
       });
     }
   }
+
+  /**
+   * The focus ring, captured while actually focused.
+   *
+   * The full-page shots above can't show this: nothing is focused in them, so
+   * a regression that removed the ring entirely would leave them all green.
+   * Keyboard focus specifically, because `:focus-visible` deliberately does
+   * not match a mouse click.
+   */
+  test(`${app.name} — focus ring is visible`, async ({ page }) => {
+    await page.setViewportSize({ width: 1440, height: 900 });
+    await gotoGallery(page, app.url, 'light');
+
+    const section = page.locator('section, div').filter({ hasText: 'Focus states' }).last();
+    await section.scrollIntoViewIfNeeded();
+
+    const button = page.getByRole('button', { name: 'Button', exact: true });
+    await button.focus();
+    await expect(button).toBeFocused();
+
+    // Assert the ring in computed styles as well as pixels. A screenshot alone
+    // would still pass if the outline came from somewhere unintended, and a
+    // token that failed to resolve leaves an empty string rather than an error.
+    const ring = await button.evaluate((el) => {
+      const s = getComputedStyle(el);
+      return {
+        width: s.outlineWidth,
+        style: s.outlineStyle,
+        token: getComputedStyle(document.documentElement)
+          .getPropertyValue('--ema-focus-ring')
+          .trim(),
+      };
+    });
+    expect(ring.style).toBe('solid');
+    expect(ring.width).toBe('2px');
+    expect(ring.token).not.toBe('');
+
+    await expect(button).toHaveScreenshot(`${app.name}-focus-ring.png`);
+  });
 }