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.

app/deck/[id]/page.tsx

@@ -6,7 +6,7 @@ import { useEffect, useState } from "react";
 
 import { ApiKeyGate } from "@/components/ApiKeyGate";
 import { StudyFeed } from "@/components/StudyFeed";
-import { Button, Spinner } from "@/components/ui";
+import { Button } from "@/components/ui";
 import { useI18n } from "@/lib/i18n";
 import type { Deck } from "@/lib/schemas";
 import { getDeck } from "@/lib/storage";
@@ -29,9 +29,15 @@ export default function DeckPage() {
 
   if (!loaded) {
     return (
-      <main className="frost-bg grid min-h-dvh place-items-center">
-        <div className="relative flex items-center gap-3 text-sm font-semibold text-frost-800 dark:text-frost-100">
-          <Spinner className="size-5" /> {t("deckPage.loading")}
+      <main className="frost-bg grid min-h-dvh place-items-center px-4">
+        <div className="w-full max-w-xl space-y-4" aria-busy="true" aria-label={t("deckPage.loading")}>
+          <div className="flex items-center gap-3">
+            <div className="skeleton size-9 rounded-xl" />
+            <div className="skeleton h-2 flex-1 rounded-full" />
+          </div>
+          <div className="skeleton h-24 rounded-3xl" />
+          <div className="skeleton h-12 rounded-2xl" />
+          <div className="skeleton h-12 w-2/3 rounded-2xl" />
         </div>
       </main>
     );