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/index.html

@@ -6,6 +6,14 @@
 <meta name="color-scheme" content="light">
 <title>Notas — editor estilo Mousepad</title>
 <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16'><rect width='16' height='16' rx='2' fill='%233584e4'/><path d='M3 4h10M3 7h10M3 10h7' stroke='white' stroke-width='1.5' stroke-linecap='round'/></svg>">
+<link rel="manifest" href="/manifest.webmanifest">
+<meta name="theme-color" content="#3584e4">
+<meta name="mobile-web-app-capable" content="yes">
+<meta name="application-name" content="Notas">
+<meta name="apple-mobile-web-app-capable" content="yes">
+<meta name="apple-mobile-web-app-status-bar-style" content="default">
+<meta name="apple-mobile-web-app-title" content="Notas">
+<link rel="apple-touch-icon" href="/icons/icon-192.png">
 <link rel="stylesheet" href="/css/app.css">
 </head>
 <body>
@@ -171,6 +179,7 @@
     <div class="menu" data-menu="ajuda">
       <button type="button" class="menu-btn">Ajuda</button>
       <ul class="menu-dropdown">
+        <li data-action="install" class="hidden">Instalar Aplicativo…</li>
         <li data-action="about">Sobre o Notas</li>
         <li data-action="logout">Sair (esquecer token)</li>
       </ul>