Replace missing install prompt with app handoff

public/manifest.webmanifest

@@ -2,7 +2,7 @@
   "name": "L484 Membership",
   "short_name": "L484",
   "description": "Private L484 membership card and agreement portal.",
-  "start_url": "/",
+  "start_url": "/?signup=continue",
   "scope": "/",
   "id": "/",
   "display": "standalone",

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE_NAME = 'l484-pwa-v7'
+const CACHE_NAME = 'l484-pwa-v8'
 const APP_SHELL = [
   '/',
   '/manifest.webmanifest',

src/App.vue

@@ -816,14 +816,20 @@ const handlePwaInstallPrimary = async () => {
     return
   }
 
-  if (installPlatform.value === 'desktop') {
-    formError.value = 'The browser install prompt is not available yet. Refresh once, then press Install app again.'
+  if (installPlatform.value === 'desktop' || installPlatform.value === 'android') {
+    signupStep.value = 2
+    saveSignupDraft()
+    pwaInstallMessage.value = 'L484 may already be installed on this device. Open the installed app to finish signup.'
     return
   }
 
-  formError.value = installPlatform.value === 'ios'
-    ? 'Install L484 from Safari using Share, Add to Home Screen, then reopen it from the app icon.'
-    : 'Install L484 from your browser menu or address bar, then reopen the installed app to continue.'
+  pwaInstallMessage.value = 'Install L484 from Safari using Share, Add to Home Screen, then reopen it from the app icon.'
+}
+
+const openInstalledApp = () => {
+  signupStep.value = 2
+  saveSignupDraft()
+  window.location.href = '/?signup=continue'
 }
 
 const startSignup = async () => {
@@ -2123,6 +2129,13 @@ onMounted(async () => {
   } else if (window.location.pathname === '/auth/nostr-callback') {
     resumePendingRemoteSignin()
   }
+  if (new URLSearchParams(window.location.search).get('signup') === 'continue') {
+    window.history.replaceState({}, '', '/')
+    currentPath.value = '/'
+    signupStep.value = currentMember.value ? 5 : 2
+    saveSignupDraft()
+    openSignup()
+  }
   window.addEventListener('popstate', () => {
     currentPath.value = window.location.pathname
   })
@@ -3493,6 +3506,9 @@ watch(mobileMenuOpen, (open) => {
             <p v-if="pwaInstallMessage" class="validation-message rounded border border-white/15 bg-white/5 p-3 text-sm text-white/70">
               Open the L484 app icon on your homescreen. Signup will continue at name entry.
             </p>
+            <button v-if="pwaInstallMessage" class="primary-action w-full" type="button" @click="openInstalledApp">
+              Open app
+            </button>
           </div>
 
           <div v-if="signupStep === 2" class="space-y-4">