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/src/routes/+page.svelte

@@ -660,7 +660,8 @@
 </Sheet>
 
 {#if loading}
-    <div data-testid="loading" role="status" aria-label="Loading manga">
+    <div data-testid="loading" role="status">
+        <span class="visually-hidden">Loading manga…</span>
         <MangaGridSkeleton />
     </div>
 {:else if error}
@@ -975,6 +976,18 @@
         color: var(--text-muted);
     }
 
+    .visually-hidden {
+        position: absolute;
+        width: 1px;
+        height: 1px;
+        padding: 0;
+        margin: -1px;
+        overflow: hidden;
+        clip: rect(0, 0, 0, 0);
+        white-space: nowrap;
+        border: 0;
+    }
+
     .error {
         color: var(--danger);
     }