feat(pwa): web manifest

Phase 8b Task 3. start_url is /intake/ (trailing slash) so the
browser falls back to whatever token was last visited, or to the
install-time URL on first open. The intake form is per-token anyway;
the developer will issue a fresh link to a client who closes and
re-opens the PWA after revocation.

public/manifest.webmanifest

@@ -0,0 +1,31 @@
+{
+  "name": "Submit a request",
+  "short_name": "Submit",
+  "description": "Send a request to the developer without signing in.",
+  "start_url": "/intake/",
+  "scope": "/intake/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "background_color": "#ffffff",
+  "theme_color": "#2f6dad",
+  "icons": [
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}