feat(pwa): app instalável com service worker e shell offline

- manifest.webmanifest: standalone, theme/background color, ícones 192/512
  any+maskable+monochrome (PNGs em web/icons) e atalho 'Nova nota' (?new=1)
- sw.js: cache versionado (notas-shell-v1); navegação network-first com
  fallback ao shell (offline), estáticos stale-while-revalidate, /api/* e
  SSE sempre passthrough; activate limpa caches e notifica janelas
- app.js: registro do SW, beforeinstallprompt com item 'Instalar Aplicativo…'
  no menu Ajuda (escondido em standalone/após instalar), toast de atualização,
  aviso offline/online, deep-link ?new=1
- index.html: meta tags theme-color/mobile-web-app/apple-touch-icon/manifest
- server: staticHandler com MIME application/manifest+json e
  Cache-Control no-cache para /sw.js; embed inclui manifest, sw e ícones
- README: seção PWA + orientação de serving (Go atual e equivalente Rust)

web/js/app.js

@@ -1792,6 +1792,15 @@
     'toggle-gutter': () => { prefs.gutter = !prefs.gutter; persistPrefs(); applyPrefs(); },
     'toggle-wrap': () => { prefs.wrap = !prefs.wrap; persistPrefs(); applyPrefs(); },
     'prefs': openPrefs,
+    'install': async () => {
+      if (!deferredPrompt) { toast('Use o ícone de instalação da barra de endereço.'); return; }
+      const p = deferredPrompt;
+      deferredPrompt = null;
+      refreshInstallUI();
+      p.prompt();
+      const { outcome } = await p.userChoice;
+      if (outcome === 'accepted') toast('Instalando o Notas como aplicativo…');
+    },
     'toggle-edit': () => setMode('edit'),
     'toggle-wysiwyg': () => setMode('wysiwyg'),
     'toggle-preview': () => setMode('preview'),
@@ -1938,6 +1947,44 @@
   E.authorInput.addEventListener('change', () =>
     localStorage.setItem('notas_author', E.authorInput.value.trim()));
 
+  /* ------------------------------------------------------------ PWA */
+
+  const isStandalone = () =>
+    window.matchMedia('(display-mode: standalone)').matches ||
+    window.matchMedia('(display-mode: window-controls-overlay)').matches ||
+    window.navigator.standalone === true;
+
+  let deferredPrompt = null;
+  const miInstall = document.querySelector('.menu-dropdown [data-action="install"]');
+
+  function refreshInstallUI() {
+    if (miInstall) miInstall.classList.toggle('hidden', isStandalone() || !deferredPrompt);
+  }
+
+  window.addEventListener('beforeinstallprompt', (e) => {
+    e.preventDefault(); // prompt próprio via menu Ajuda
+    deferredPrompt = e;
+    refreshInstallUI();
+  });
+  window.addEventListener('appinstalled', () => {
+    deferredPrompt = null;
+    refreshInstallUI();
+    toast('Notas instalado ✔ procure pelo menu do sistema');
+  });
+
+  if ('serviceWorker' in navigator && location.protocol.startsWith('http')) {
+    navigator.serviceWorker.register('/sw.js', { scope: '/' }).catch(() => { /* sem SW: app continua */ });
+    navigator.serviceWorker.addEventListener('message', (e) => {
+      if (e.data === 'notas:updated') toast('App atualizado para a nova versão');
+    });
+  }
+
+  window.addEventListener('offline', () => toast('Sem conexão — o app abre offline; salvar precisa do servidor'));
+  window.addEventListener('online', () => { toast('Conexão restabelecida'); checkFreshness(); });
+
+  const wantsNew = new URLSearchParams(location.search).get('new') === '1';
+  if (wantsNew) history.replaceState(null, '', location.pathname);
+
   /* ------------------------------------------------------------ boot */
 
   async function boot() {
@@ -1946,6 +1993,8 @@
     await restoreTabs();
     updateStatus();
     state.booted = true;
+    refreshInstallUI();
+    if (wantsNew) await newDraftTab();
     connectEvents();
     sendPresence();
     setInterval(sendPresence, 10000); // heartbeat de presença