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/Sidebar.tsx

@@ -18,6 +18,7 @@ import {
 import { useTheme } from "@/components/ThemeToggle";
 import { createFolder, folderStore } from "@/lib/folders";
 import { useI18n } from "@/lib/i18n";
+import { springSheet } from "@/lib/motion";
 import { ALL_FOLDERS, folderSelection, sidebarOpen, useSelectedFolder, useSidebarOpen } from "@/lib/nav";
 import { UNORGANIZED_ID } from "@/lib/schemas";
 import { deckStore } from "@/lib/storage";
@@ -416,7 +417,14 @@ export function MobileSidebar() {
             initial={{ x: -280 }}
             animate={{ x: 0 }}
             exit={{ x: -280 }}
-            transition={{ type: "spring", stiffness: 380, damping: 36 }}
+            transition={springSheet}
+            drag="x"
+            dragConstraints={{ left: -280, right: 0 }}
+            dragElastic={{ left: 0.45, right: 0 }}
+            dragMomentum={false}
+            onDragEnd={(_, gesture) => {
+              if (gesture.offset.x < -80 || gesture.velocity.x < -500) sidebarOpen.set(false);
+            }}
             className="sidebar-surface relative z-10 flex h-full w-[272px] flex-col rounded-l-none rounded-r-[1.5rem] px-2.5 py-3 pb-safe"
           >
             <NavContent onNavigate={() => sidebarOpen.set(false)} />