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.

lib/settings.ts

@@ -20,9 +20,11 @@ export type BrowserSettings = {
    * and model names always come from the server's .env, never from the client.
    */
   apiKey: string;
+  /** Best-effort haptics on supported devices (phones). On by default. */
+  haptics: boolean;
 };
 
-const EMPTY: BrowserSettings = { apiKey: "" };
+const EMPTY: BrowserSettings = { apiKey: "", haptics: true };
 
 let cached: BrowserSettings | null = null;
 const listeners = new Set<() => void>();
@@ -33,7 +35,10 @@ function read(): BrowserSettings {
     const raw = window.localStorage.getItem(STORAGE_KEY);
     if (!raw) return EMPTY;
     const parsed = JSON.parse(raw) as Partial<BrowserSettings>;
-    return { apiKey: typeof parsed.apiKey === "string" ? parsed.apiKey.slice(0, 512) : "" };
+    return {
+      apiKey: typeof parsed.apiKey === "string" ? parsed.apiKey.slice(0, 512) : "",
+      haptics: parsed.haptics !== false,
+    };
   } catch {
     return EMPTY;
   }
@@ -42,8 +47,8 @@ function read(): BrowserSettings {
 function write(next: BrowserSettings): void {
   cached = next;
   try {
-    if (next.apiKey) {
-      window.localStorage.setItem(STORAGE_KEY, JSON.stringify({ apiKey: next.apiKey }));
+    if (next.apiKey || next.haptics === false) {
+      window.localStorage.setItem(STORAGE_KEY, JSON.stringify({ apiKey: next.apiKey, haptics: next.haptics }));
     } else {
       window.localStorage.removeItem(STORAGE_KEY);
     }
@@ -72,12 +77,13 @@ export function getApiKey(): string {
   return settingsStore.getSnapshot().apiKey;
 }
 
-export function saveSettings(next: BrowserSettings): void {
-  write(next);
+/** Merge-update: callers only pass what they changed. */
+export function saveSettings(next: Partial<BrowserSettings>): void {
+  write({ ...settingsStore.getSnapshot(), ...next });
 }
 
 export function clearApiKey(): void {
-  write({ apiKey: "" });
+  write({ ...settingsStore.getSnapshot(), apiKey: "" });
 }
 
 /** Header used to hand the key to our own API routes for one request. */