feat(web): home story-card polish + play page back-link rebrand

Home (apps/web/app/page.tsx):
- StoryCard locked to uniform aspectRatio "4 / 5". The previous
  "placeholder 4/5 → naturalRatio after onLoad" flow coupled card
  height to lazy-load order: cards still below the fold sat at the
  placeholder ratio while above-the-fold cards snapped to their
  image's actual ratio (1.6 landscape vs 0.75 portrait vs 1.23
  squarish), so the gallery looked inconsistent until a hard refresh
  re-decoded everything from cache synchronously. Fixed ratio +
  object-cover removes the coupling.
- StoryCard hover overlay collapsed from two sibling layers
  (backdrop-blur + mask-image + dark gradient sibling) into one
  element with a pure rgba(0,0,0,…) linear-gradient and an opacity
  transition. Chromium does not animate backdrop-filter cleanly when
  combined with mask-image on an empty element — the first hover
  frame shows a full rectangular blur before the mask kicks in, then
  snaps to the feathered shape ("矩形磨砂 → 渐变磨砂"). One layer,
  one transitioning property, no compositing race.

Play (apps/web/app/play/page.tsx):
- Header back-link "云梦" → "InfiPlot" using the same serif + italic
  ember "Plot" treatment as the homepage wordmark. Resolved against
  the parallel plain-text rebrand already on infiplot/staging by
  keeping the styled version for brand consistency.

apps/web/app/play/page.tsx

@@ -890,10 +890,12 @@ function PlayInner() {
       <header className="px-5 md:px-12 pt-6 md:pt-8 flex items-center justify-between">
         <Link
           href="/"
-          className="text-[10px] smallcaps text-clay-600 hover:text-clay-900 transition-colors flex items-center gap-2"
+          className="text-clay-600 hover:text-clay-900 transition-colors flex items-center gap-2"
         >
           <i className="fa-solid fa-arrow-left text-[9px]" />
-          InfiPlot
+          <span className="font-serif text-[15px] leading-none tracking-tight">
+            Infi<em className="italic font-light text-ember-500">Plot</em>
+          </span>
         </Link>
         <div className="flex items-center gap-3 text-[10px] smallcaps text-clay-500 num">
           <span>第 · {String(sceneCount).padStart(3, "0")} · 幕</span>