feat(frontend): mobile manga detail with hero + sticky CTA (0.58.0)

Phase 3 of the mobile redesign: the manga detail page gets a mobile-
native chrome — blurred-cover hero with a transparent app bar overlay,
secondary actions in an overflow Sheet, a 3-line description clamp
with Read more, and a sticky bottom CTA whose wording reflects read
progress. Desktop layout is untouched.

- /manga/[id] joins the layout's HIDE_MOBILE_CHROME route set so the
  global AppBar and BottomNav step aside for the page-specific hero
  chrome. A new `html[data-mobile-full-bleed]` opt-in zeroes main's
  mobile top padding without touching horizontal/bottom gutters.
- The hero block (mobile-only via CSS) renders a blurred cover
  backdrop, a transparent app bar (back / bookmark / overflow ⋯), and
  a sharp cover thumbnail beside the title, authors, and status. The
  bookmark icon swaps between Bookmark and BookmarkCheck based on the
  current bookmark state — reusing the existing toggleBookmark flow.
- Secondary actions (Add to collection / Edit / Upload chapter /
  Force resync) move into an overflow Sheet opened from the ⋯
  button. The desktop action-row stays.
- Description gets a `.clamped` modifier (-webkit-line-clamp: 3) and
  a Read more toggle on mobile, gated by a >200-char heuristic so
  short blurbs don't render a dangling button. Desktop shows the
  full description as before.
- Sticky bottom CTA bar reads "Continue {chapterLabel}" when
  data.readProgress has a chapter_id, "Read first chapter" when
  the manga has chapters but no progress, and hides otherwise. The
  bar honors env(safe-area-inset-bottom) so it sits above the iOS
  home indicator.
- New Playwright spec covers the CTA wording state machine
  (no-progress → Read first chapter, has-progress → Continue),
  Read more expand/collapse, overflow sheet contents, and a desktop
  regression for the unchanged action-row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -87,9 +87,13 @@
     // is unaffected; it continues to follow the reader fullscreen slide
     // transform.
     const HIDE_MOBILE_CHROME_PATHS = new Set(['/login', '/register']);
+    const HIDE_MOBILE_CHROME_ROUTES = new Set([
+        '/manga/[id]',
+        '/manga/[id]/chapter/[chapter_id]'
+    ]);
     const showMobileChrome = $derived.by(() => {
         if (HIDE_MOBILE_CHROME_PATHS.has($page.url.pathname)) return false;
-        if ($page.route?.id === '/manga/[id]/chapter/[chapter_id]') return false;
+        if (HIDE_MOBILE_CHROME_ROUTES.has($page.route?.id ?? '')) return false;
         return true;
     });
 
@@ -381,6 +385,18 @@
         padding-top: 0;
     }
 
+    @media (max-width: 640px) {
+        :global(html[data-mobile-full-bleed='true']) main {
+            /* Mobile detail page surrenders the layout's top reservation
+               so its hero can sit at the viewport top under a transparent
+               page-specific app bar. Horizontal and bottom padding are
+               preserved so the body content (description, chapter list)
+               keeps its gutters; the hero negates the horizontal padding
+               itself via negative margins. */
+            padding-top: 0;
+        }
+    }
+
     /* Mobile chrome (AppBar + BottomNav) is rendered always when the
        route opts in, but CSS-hides above the breakpoint so the desktop
        header is the only visible top-chrome. The ResizeObserver in the