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/manifest.webmanifest

@@ -0,0 +1,19 @@
+{
+  "name": "Web Terminal",
+  "short_name": "WebTerm",
+  "description": "Drive your shell / Claude Code from any device on your LAN.",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#1a1a1a",
+  "theme_color": "#1a1a1a",
+  "icons": [
+    {
+      "src": "./icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}