Native-feel pass: PWA integration, system back, offline, haptics, undo

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

System integration
- Manifest: id, display_override, monochrome icon, Make my homework
  shortcut and a share target (text/URL via GET, files via POST).
  iOS startup images for the common iPhone sizes. Pinch-zoom is allowed
  again (double-tap zoom is disabled via touch-action instead).
- Share target: /share chooser with New deck / Make my homework; POSTed
  files wait in a 10-minute in-memory store, reach the wizard through
  sessionStorage and never touch disk.
- Web Share: Share rows for decks (JSON file where supported) and the
  homework result, with a clipboard fallback and a toast.
- Offline: a quiet pill appears and Freeze / mode cards are disabled
  with a clear reason; input is never lost (error cards keep Retry).
- State: wizard drafts (text, options, photos up to a size budget)
  survive reloads and OS kills and are cleared on success; re-tapping
  the active tab scrolls to the top; the deck page shows a skeleton
  instead of a spinner.

Navigation and gestures
- System Back closes the topmost layer first: a small history-entry
  stack (lib/use-back-close.ts) covers the drawer, Settings sheet,
  install sheet, deck actions panel and one wizard step; navigations
  are never rewound and stale entries are skipped.
- Destructive-but-reversible actions use Undo toasts: deleting a deck
  (no confirm) and removing a file in the wizard. Irreversible ones
  keep their confirm.

Input and feedback
- Every phone input got inputmode / enterkeyhint / autocomplete /
  autocapitalize / spellcheck; Enter on the last field advances the
  wizard; a clipboard-paste button appears where the API exists.
- Haptics behind a Settings toggle (on by default): tab switches,
  toggles, correct/wrong, completion; Vibration API + the iOS hidden
  switch tick, throttled and silent everywhere else.
- Wake lock during the study feed and homework guide, released on
  leave and re-acquired when the tab returns.

Verified: new native suite 35/35 (manifest/head/viewport, offline,
share target GET + POST, back-gesture layers, undo, share fallback,
tab re-tap, haptics persistence, draft restore), regression 74/74,
mobile 52/52, wizard 80/80, declutter 116/116, animation 8/8,
i18n 25/25. Lint + build clean. SW freezmo-v23.

Needs a real device (documented in the README): iOS haptic tick,
system share sheet entry, wake-lock grant, iPhone startup images,
iOS edge-swipe back, install prompts. Pull-to-refresh and the app
badge were skipped on purpose.

components/Sidebar.tsx

@@ -17,12 +17,14 @@ import {
 } from "@/components/icons";
 import { useTheme } from "@/components/ThemeToggle";
 import { createFolder, folderStore } from "@/lib/folders";
+import { haptic } from "@/lib/haptics";
 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";
 import { openSettings } from "@/lib/ui-events";
+import { useBackClose } from "@/lib/use-back-close";
 import { cn } from "@/lib/utils";
 
 const COLLAPSE_KEY = "freezmo.sidebarCollapsed";
@@ -303,7 +305,10 @@ function NavContent({
         </button>
         <button
           type="button"
-          onClick={toggle}
+          onClick={() => {
+            haptic("toggle");
+            toggle();
+          }}
           aria-label={theme === "dark" ? t("common.switchToLight") : t("common.switchToDark")}
           title={theme === "dark" ? t("common.lightMode") : t("common.darkMode")}
           className={cn("nav-row", collapsed && "justify-center px-2")}
@@ -396,6 +401,8 @@ export function Sidebar() {
 export function MobileSidebar() {
   const open = useSidebarOpen();
   const { t } = useI18n();
+  /* Back closes the drawer before it navigates away. */
+  const { skipRewind } = useBackClose(open, () => sidebarOpen.set(false));
 
   return (
     <AnimatePresence>
@@ -427,7 +434,12 @@ export function MobileSidebar() {
             }}
             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)} />
+            <NavContent
+              onNavigate={() => {
+                skipRewind();
+                sidebarOpen.set(false);
+              }}
+            />
           </motion.aside>
         </motion.div>
       ) : null}