feat: add NIP-55 Android signer support for Amber/Primal login

When window.nostr isn't available (common on mobile Chrome where
extensions can't inject), fall back to NIP-55 nostrsigner: intent
URIs. This opens Amber/Primal directly to sign a NIP-98 event,
then redirects back with the signed event for JWT authentication.

- Build nostrsigner: URI with unsigned NIP-98 event + callback URL
- Process NIP-55 callback on page mount (extract signed event from URL)
- Auto-detect Android to show "SIGN IN WITH AMBER / PRIMAL" label
- Reduced window.nostr polling from 3s to 2s before NIP-55 fallback

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/JoinBoutPage.vue

@@ -10,7 +10,7 @@ import WalletConnect from '../components/WalletConnect.vue'
 import { authFetch } from '../lib/nostr-auth'
 
 const router = useRouter()
-const { pubkey, bot, profilePicUrl, isLoggedIn, hasExtension, hasStoredKey, isLoading, login, waitForSigner, generateLogin, loginWithNsec, registerBot, registerHuman, getStoredNsec, logout } = useNostr()
+const { pubkey, bot, profilePicUrl, isLoggedIn, hasExtension, hasStoredKey, isLoading, login, waitForSigner, generateLogin, loginWithNsec, registerBot, registerHuman, getStoredNsec, logout, initiateNip55Login, processNip55Return, hasAndroidSigner } = useNostr()
 const showNsecBackup = ref(false)
 const generatedNsec = ref('')
 const nsecInput = ref('')
@@ -131,7 +131,25 @@ function startRateLimitTimer(msg: string) {
   }, 1000)
 }
 
-onMounted(() => {
+onMounted(async () => {
+  // Check for NIP-55 callback (returning from Amber/Primal signer)
+  try {
+    const nip55Result = await processNip55Return()
+    if (nip55Result) {
+      if (nip55Result.bot) {
+        isHumanMode.value = !!nip55Result.bot.isHuman
+        step.value = 'ready'
+      } else {
+        step.value = 'choose-mode'
+      }
+      pollQueue()
+      pollHandle = setInterval(pollQueue, 3000)
+      return
+    }
+  } catch (e) {
+    error.value = 'Signer login failed. Try again.'
+  }
+
   // If already logged in with a bot, go straight to ready
   if (isLoggedIn.value) {
     step.value = 'ready'
@@ -189,8 +207,14 @@ async function handleSignerLogin() {
   if (!window.nostr) {
     // Mobile signers inject late — wait briefly
     error.value = 'Looking for signer...'
-    const found = await waitForSigner(3000)
+    const found = await waitForSigner(2000)
     if (!found) {
+      // On Android, try NIP-55 intent to open Amber/Primal directly
+      if (hasAndroidSigner.value) {
+        error.value = 'Opening signer app...'
+        initiateNip55Login()
+        return
+      }
       error.value = 'No Nostr signer detected. Install a NIP-07 extension (Nos2x, Alby) or use Amber on Android.'
       return
     }
@@ -588,7 +612,7 @@ function handleSignOut() {
             @click="handleSignerLogin"
           >
             <span v-if="isLoading" class="w-5 h-5 border-2 border-neon-yellow/30 border-t-neon-yellow rounded-full animate-spin" />
-            {{ isLoading ? 'CONNECTING...' : 'USE NOSTR SIGNER' }}
+            {{ isLoading ? 'CONNECTING...' : hasAndroidSigner ? 'SIGN IN WITH AMBER / PRIMAL' : 'USE NOSTR SIGNER' }}
           </button>
 
           <!-- Sign in with stored key -->