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>

src/motion/variants.ts

@@ -0,0 +1,58 @@
+import type { Transition, Variants } from "framer-motion";
+
+/** Premium ease-out curve shared by all motion. */
+export const EASE_OUT = [0.22, 1, 0.36, 1] as const;
+
+/** Base transition for reveal-style animations. */
+export const baseTransition: Transition = {
+  duration: 0.4,
+  ease: EASE_OUT,
+};
+
+/**
+ * Fade + lift in. `visible` is a function variant so callers can pass a
+ * per-item delay via `custom` (defaults to 0, e.g. when driven by a
+ * stagger container).
+ */
+export const fadeInUp: Variants = {
+  hidden: { opacity: 0, y: 16 },
+  visible: (delay: number = 0) => ({
+    opacity: 1,
+    y: 0,
+    transition: { ...baseTransition, delay },
+  }),
+};
+
+/** Fade + subtle scale in (cards, popovers). */
+export const scaleIn: Variants = {
+  hidden: { opacity: 0, scale: 0.96 },
+  visible: (delay: number = 0) => ({
+    opacity: 1,
+    scale: 1,
+    transition: { ...baseTransition, delay },
+  }),
+};
+
+/** Parent container that staggers its children's `visible` state. */
+export const staggerContainer: Variants = {
+  hidden: {},
+  visible: {
+    transition: { staggerChildren: 0.06, delayChildren: 0.02 },
+  },
+};
+
+/** Route enter/exit transition used with AnimatePresence. */
+export const pageTransition: Variants = {
+  initial: { opacity: 0, y: 8 },
+  enter: { opacity: 1, y: 0, transition: { duration: 0.24, ease: EASE_OUT } },
+  exit: { opacity: 0, y: -6, transition: { duration: 0.16, ease: EASE_OUT } },
+};
+
+/** Springy hover lift for cards. Use rest/hover states on an `m` element. */
+export const cardHover: Variants = {
+  rest: { y: 0 },
+  hover: {
+    y: -4,
+    transition: { type: "spring", stiffness: 380, damping: 26 },
+  },
+};