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.

app/create/page.tsx

@@ -2,6 +2,7 @@ import { redirect } from "next/navigation";
 
 import { ApiKeyGate } from "@/components/ApiKeyGate";
 import { AppHeader } from "@/components/AppHeader";
+import { RouteTransition } from "@/components/RouteTransition";
 import { MobileSidebar, Sidebar, SidebarOffset } from "@/components/Sidebar";
 import { TabBar } from "@/components/TabBar";
 import { UploadScreen } from "@/components/UploadScreen";
@@ -29,7 +30,9 @@ export default async function CreatePage({
         <main className="flex min-h-dvh min-w-0 flex-1 flex-col">
           <AppHeader />
           <div className="mx-auto w-full max-w-3xl flex-1 px-4 pb-tabbar lg:px-10 lg:pb-16">
-            <UploadScreen />
+            <RouteTransition>
+              <UploadScreen />
+            </RouteTransition>
           </div>
           <TabBar />
         </main>