fix(a11y): add a skip link, demote the subtitle, and size the sort arrow

a11y.30: `<main id="main-content">` existed and nothing linked to it,
so a keyboard user walked the library filter, the search box, the job
indicator and eleven nav items before reaching content, on every
navigation. Two things in it are load-bearing and only checkable
against the running document: the link is out of flow in *both* states,
because `body` is a grid with named areas and an in-flow extra child is
auto-placed into one of them; and `<main>` needs tabindex="-1", or the
fragment link moves the scroll, leaves the tab sequence where it was,
and looks like it worked.

a11y.29: `<h1>` followed by `<h3>` for type size. An `hgroup` takes one
heading plus paragraphs, so a `<p>` is also what it was meant to hold.

a11y.34: the sort arrow was 10px, below the type scale's own floor,
with a comment acknowledging it. Half of that finding was closed by
Phase 1 — the direction is announced now, via aria-sort — and the other
half is one declaration.

And the state that landed in: the hgroup measured 67px inside a 64px
bar, so dropping the h3's bottom margin shortened the block, moved the
flex-centred pair down, and clipped the subtitle's descenders. The
overflow was pre-existing; `margin-block: 0` on the title is the fix,
pinned by a new layout-overflow case.

e2e/specs/layout-overflow.spec.ts

@@ -135,3 +135,26 @@ test.describe('the app fits in its own window', () => {
     expect(reachable).toBe(true);
   });
 });
+
+test.describe('the title block fits its bar', () => {
+  test('the hgroup stays inside the 4em top bar', async ({ app }) => {
+    // The state a11y.29 landed in. The pair is flex-centred and a UA
+    // gives an `h1` a 0.67em top margin, so the block measured 67px
+    // inside 64 — pre-existing, and invisible until dropping the h3's
+    // bottom margin shortened the block and shifted it down into the
+    // clip. The descenders of "meant to bee." were cut.
+    const fits = await app.locator('hgroup').evaluate((el) => {
+      const bar = el.closest('.top-bar')!.getBoundingClientRect();
+      const group = el.getBoundingClientRect();
+
+      return {
+        top: group.top >= Math.floor(bar.top),
+        bottom: group.bottom <= Math.ceil(bar.bottom),
+        height: Math.round(group.height),
+      };
+    });
+
+    expect(fits.top).toBe(true);
+    expect(fits.bottom).toBe(true);
+  });
+});