feat(v0.3): QR connect (M5) + PWA installable (M4)

- M5: 📱 toolbar button → modal with a client-side QR of location.origin
  (shareable when opened via the LAN IP) + localhost tip
- M4: manifest.webmanifest + icon.svg + sw.js (network-first, never intercepts
  /term or /hook) + registration in main.ts; apple-touch + theme-color meta
- Browser-verified: QR canvas renders, SW 'controlled', manifest linked,
  no console errors. 199 tests green, build ok.

public/main.ts

@@ -14,6 +14,7 @@ import '@xterm/xterm/css/xterm.css'
 import { mountKeybar } from './keybar.js'
 import { TabApp } from './tabs.js'
 import { mountSearch } from './search.js'
+import { mountQrConnect } from './qr.js'
 
 const paneHost = document.getElementById('term')
 const tabs = document.getElementById('tabs')
@@ -32,3 +33,13 @@ mountSearch(toolbar, {
   find: (query, dir) => app.findInActive(query, dir),
   clear: () => app.clearActiveSearch(),
 })
+mountQrConnect(toolbar)
+
+// PWA: register the service worker (installable + offline shell, M4).
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    void navigator.serviceWorker.register('./sw.js').catch(() => {
+      // SW registration is best-effort; the app works without it.
+    })
+  })
+}