Animation pass: one shared motion language for phones

Mobile only (<=639px / coarse pointers); desktop keeps its behaviour.

Foundation
- lib/motion.ts owns the spring presets (snappy/gentle/sheet), durations,
  easings and variants (screens, tabs, wizard steps, list stagger).
- Native-feel CSS: press feedback scales every button, card, chip, tab,
  row and summary to 0.96 with a spring-like cubic-bezier on transform
  (scoped to phones/coarse pointers), tap-highlight off, touch-action
  manipulation, chrome text non-selectable, reduced motion drops it.
- App shell fades in once on launch; the study feed fades in on phones.

Applied
- Tab bar: sliding active pill (shared layoutId) + icon bounce.
- Study Decks/Homework, language toggle, difficulty and size pickers:
  the same gliding highlight, gated so desktop keeps the old styling.
- Route content enters direction-aware (deeper = from the right, back =
  from the left, same-depth = crossfade) via RouteTransition on the
  library/create/homework content columns; library tab switches
  crossfade; wizard steps slide forward/backward (reduced motion safe).
- Library: cards fade up in a capped stagger on first load, glide with
  layout animations and shrink away on delete (AnimatePresence).
- Study feed: correct feedback pulses, wrong shakes, the progress bar
  settles with a spring curve, score and XP count up on the results.
- Settings sheet uses the sheet spring; the drawer now follows the
  finger and closes on release velocity, like the sheet.
- Generation progress: the stage message fades/slides between stages;
  the icon keeps its gentle wobble; the feed fades in on hand-off.

Skipped, on purpose: LazyMotion (the sliding highlights need layout
animations = domMax, same features the current import already has);
route exit animations (the App Router unmounts the outgoing page —
exits are handled by in-page AnimatePresence); replacing the study
feed's native scroll-snap with finger-follow drag (would fight the
platform scroller); finger-follow height animation for native <details>
expanders.

Verified: animation suite 8/8 (reduced motion + 4x-CPU-throttled trace:
no long tasks, CLS 0.0000), regression 74/74, mobile 52/52, declutter
116/116, i18n 25/25. Lint + build clean. SW freezmo-v21.

components/SegmentedPill.tsx

@@ -0,0 +1,36 @@
+"use client";
+
+import { motion, useReducedMotion } from "framer-motion";
+
+import { springSnappy } from "@/lib/motion";
+import { cn } from "@/lib/utils";
+
+/**
+ * The gliding highlight shared by segmented controls and pickers. One
+ * `layoutId` per control makes the pill slide between its items; under
+ * reduced motion it simply appears in place.
+ */
+export function SegmentedPill({
+  layoutId,
+  className,
+  plain,
+}: {
+  layoutId: string;
+  className?: string;
+  /** Skip the ice gradient (for pickers that keep their own selected look). */
+  plain?: boolean;
+}) {
+  const reduceMotion = useReducedMotion();
+  const base = plain ? "slide-pill" : "slide-pill grad-ice shadow-[0_6px_16px_-8px_rgb(56_189_248/0.7)]";
+  if (reduceMotion) {
+    return <span aria-hidden className={cn(base, className)} />;
+  }
+  return (
+    <motion.span
+      aria-hidden
+      layoutId={layoutId}
+      transition={springSnappy}
+      className={cn(base, className)}
+    />
+  );
+}