feat: add KeysModal for managing private key accounts

- Introduced a new KeysModal component to display and manage nsec/npub for accounts with local private keys.
- Updated AppHeader and Profile views to include a "My Keys" button, conditionally rendered based on the presence of a private key.
- Enhanced the useAccounts composable to determine if the active account holds a local private key, enabling key management functionality.

These changes improve user access to their private key information and enhance the overall account management experience.

src/composables/useAccounts.ts

@@ -273,6 +273,48 @@ export function useAccounts() {
     subscriptions.forEach((sub) => sub.unsubscribe())
   })
 
+  /**
+   * Whether the active account holds a local private key
+   * (generated sovereign identity or imported nsec).
+   * When true, the user can view/export their keys.
+   */
+  const hasPrivateKey = computed(() => {
+    const acct = activeAccount.value
+    if (!acct) return false
+    // PrivateKeyAccount has type 'local' and stores the secret key
+    // in its signer. Check the account type name set by applesauce.
+    const typeName = acct.constructor?.name ?? acct.type ?? ''
+    return typeName === 'PrivateKeyAccount' || typeName === 'local'
+  })
+
+  /**
+   * Retrieve the active account's nsec and npub.
+   * Only works for private-key-based accounts (generated or imported nsec).
+   * Returns null if the active account doesn't hold a local secret key.
+   */
+  async function getAccountKeys(): Promise<{ nsec: string; npub: string; hexPub: string } | null> {
+    const acct = activeAccount.value
+    if (!acct?.pubkey) return null
+
+    try {
+      const nip19 = await import('nostr-tools/nip19')
+      const npub = nip19.npubEncode(acct.pubkey)
+
+      // The PrivateKeyAccount stores the secret key in its signer.
+      // Access it through the account's serialisation (toJSON includes secrets).
+      const signer = acct.signer ?? acct._signer
+      const secretKey: Uint8Array | undefined = signer?.key ?? signer?.secretKey ?? signer?._key
+
+      if (!secretKey) return null
+
+      const nsec = nip19.nsecEncode(secretKey)
+      return { nsec, npub, hexPub: acct.pubkey }
+    } catch (err) {
+      console.warn('[useAccounts] Failed to extract keys:', err)
+      return null
+    }
+  }
+
   return {
     // State
     activeAccount,
@@ -285,6 +327,10 @@ export function useAccounts() {
     activeProfile,
     activeProfilePicture,
 
+    // Key management
+    hasPrivateKey,
+    getAccountKeys,
+
     // Login methods
     loginWithExtension,
     loginWithPersona,