fix: address skeleton/nav-progress review findings

Correctness:
- MangaGridSkeleton: covers rendered 0px tall because height="0" is a
  definite height that defeats aspect-ratio. Skeleton gains an `aspectRatio`
  prop (height stays auto) so the 2/3 cover box is reserved and the grid no
  longer shifts when real covers load.
- Move `.manga-card` layout into global tokens.css (it was scoped to
  MangaCard, so the skeleton's cards inherited none of it). Both consumers
  now share one definition.
- NavProgress: rebuilt as an idle/loading/done state machine. It now
  completes to full and fades on settle (was snapping away instantly),
  restarts the trickle when one navigation supersedes another (was parking
  frozen near 90%), animates transform/opacity only with will-change gated to
  the active phase, and hides itself in reader fullscreen.
- Skeleton: text-variant height resolved in one place (no inline style
  shadowing a stylesheet rule); shimmer gradient/animation gated behind
  prefers-reduced-motion: no-preference so reduced motion shows a flat block.

A11y:
- Home loading region announces via visually-hidden text instead of an
  aria-label a live region won't reliably read.

Cleanup:
- Drop the fragile :first-child selector and index-keying ceremony in
  MangaGridSkeleton; simplify Skeleton's style construction.

Tests: adds regression guards for the 0px cover collapse, the aspect-ratio
box, text-height defaulting, and the nav-progress complete/restart phases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/e2e/nav-progress.spec.ts

@@ -132,16 +132,17 @@ test('shows the nav progress bar during a pending navigation and clears it after
 
     const bar = page.getByTestId('nav-progress');
     // Idle on a settled page.
-    await expect(bar).not.toHaveClass(/active/);
+    await expect(bar).toHaveAttribute('data-phase', 'idle');
 
     // Client-side navigate into the (gated) detail page.
     await page.locator(`a[href="/manga/${mangaId}"]`).first().click();
 
-    // Bar activates while the detail load is in flight.
-    await expect(bar).toHaveClass(/active/);
+    // Bar enters the loading phase while the detail load is in flight.
+    await expect(bar).toHaveAttribute('data-phase', 'loading');
 
-    // Let the load resolve; the detail renders and the bar goes idle again.
+    // Let the load resolve; the detail renders and the bar settles back to
+    // idle (via a brief done phase).
     releaseDetail();
     await expect(page.getByTestId('manga-title')).toHaveText('Berserk');
-    await expect(bar).not.toHaveClass(/active/);
+    await expect(bar).toHaveAttribute('data-phase', 'idle');
 });