feat(app): add Nostr identity login via NIP-07 browser extension

Create useNostrIdentity.ts composable with NIP-07 window.nostr API
integration (getPublicKey, signEvent, extension detection). Add npub
bech32 encoding to bech32.ts. Create NostrLogin.vue settings component
with login/logout, npub display with copy, and extension install guidance.

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

packages/app/src/composables/useNostrIdentity.ts

@@ -0,0 +1,119 @@
+import { ref, computed, onMounted } from 'vue'
+import { encodeNpub } from '@/utils/bech32'
+
+/**
+ * NIP-07 browser extension interface (nos2x, Alby, etc.)
+ */
+interface NostrExtension {
+  getPublicKey(): Promise<string>
+  signEvent(event: UnsignedNostrEvent): Promise<SignedNostrEvent>
+  getRelays?(): Promise<Record<string, { read: boolean; write: boolean }>>
+  nip04?: {
+    encrypt(pubkey: string, plaintext: string): Promise<string>
+    decrypt(pubkey: string, ciphertext: string): Promise<string>
+  }
+}
+
+export interface UnsignedNostrEvent {
+  kind: number
+  created_at: number
+  tags: string[][]
+  content: string
+}
+
+export interface SignedNostrEvent extends UnsignedNostrEvent {
+  id: string
+  pubkey: string
+  sig: string
+}
+
+declare global {
+  interface Window {
+    nostr?: NostrExtension
+  }
+}
+
+const pubkey = ref<string | null>(null)
+const isAvailable = ref(false)
+const isLoading = ref(false)
+const error = ref<string | null>(null)
+
+export function useNostrIdentity() {
+  const npub = computed(() => {
+    if (!pubkey.value) return null
+    try {
+      return encodeNpub(pubkey.value)
+    } catch {
+      return null
+    }
+  })
+
+  const isLoggedIn = computed(() => !!pubkey.value)
+
+  const truncatedNpub = computed(() => {
+    if (!npub.value) return null
+    return npub.value.slice(0, 12) + '...' + npub.value.slice(-8)
+  })
+
+  function checkAvailability() {
+    isAvailable.value = typeof window !== 'undefined' && !!window.nostr
+  }
+
+  async function login(): Promise<void> {
+    error.value = null
+    if (!window.nostr) {
+      error.value = 'No Nostr extension detected. Install nos2x, Alby, or another NIP-07 extension.'
+      return
+    }
+
+    isLoading.value = true
+    try {
+      const pk = await window.nostr.getPublicKey()
+      pubkey.value = pk
+    } catch (e) {
+      error.value = e instanceof Error ? e.message : 'Failed to get public key'
+    } finally {
+      isLoading.value = false
+    }
+  }
+
+  async function signEvent(event: UnsignedNostrEvent): Promise<SignedNostrEvent | null> {
+    error.value = null
+    if (!window.nostr) {
+      error.value = 'No Nostr extension detected'
+      return null
+    }
+
+    try {
+      return await window.nostr.signEvent(event)
+    } catch (e) {
+      error.value = e instanceof Error ? e.message : 'Failed to sign event'
+      return null
+    }
+  }
+
+  function logout() {
+    pubkey.value = null
+    error.value = null
+  }
+
+  onMounted(() => {
+    checkAvailability()
+    // Some extensions load async — recheck after a short delay
+    setTimeout(checkAvailability, 500)
+  })
+
+  return {
+    pubkey,
+    npub,
+    isAvailable,
+    isLoggedIn,
+    isLoading,
+    error,
+    truncatedNpub,
+    login,
+    logout,
+    signEvent,
+    checkAvailability,
+  }
+}