feat: scroll to post bubble from recommended card + back-to-top button

Recommended cards already routed to /browse#post-<id>, but the stream had
no logic to scroll to the target bubble — and the post might not be paged
in yet. MessageStream now resolves the #post-<id> hash, auto-loads more
pages until the bubble renders, scrolls to it, and gives it a brief gold
highlight. Bubbles get scroll-mt so they clear the sticky header.

Also adds a global floating back-to-top button (BackToTop) mounted in
PublicLayout, shown after scrolling past 400px.

Bundles related staging UI work already present in the working tree.

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

tailwind.config.js

@@ -14,6 +14,24 @@ export default {
           muted: "#8f9099",
         },
       },
+      keyframes: {
+        shimmer: {
+          "100%": { transform: "translateX(100%)" },
+        },
+        "fade-in-up": {
+          from: { opacity: "0", transform: "translateY(16px)" },
+          to: { opacity: "1", transform: "translateY(0)" },
+        },
+        "scale-in": {
+          from: { opacity: "0", transform: "scale(0.96)" },
+          to: { opacity: "1", transform: "scale(1)" },
+        },
+      },
+      animation: {
+        shimmer: "shimmer 1.5s infinite",
+        "fade-in-up": "fade-in-up 0.5s cubic-bezier(0.22, 1, 0.36, 1) both",
+        "scale-in": "scale-in 0.4s cubic-bezier(0.22, 1, 0.36, 1) both",
+      },
       fontFamily: {
         sans: [
           "Noto Sans SC",