feat: implement three-phase Amber login flow in AuthModal

- Updated Amber login button to reflect new three-phase process.
- Added handling for reading public key and signature from clipboard.
- Introduced new state management for Amber login phases.
- Enhanced user feedback during each phase with appropriate messaging.
- Refactored related functions for clarity and maintainability.

src/stores/auth.ts

@@ -243,9 +243,19 @@ export const useAuthStore = defineStore('auth', () => {
   }
 
   /**
-   * Login with Nostr signature
+   * Login with Nostr signature.
+   *
+   * @param preSignedAuthHeader  Optional pre-built `Nostr <base64>` header.
+   *   When provided the active signer is NOT called for NIP-98 signing.
+   *   This is required for Amber / clipboard-based signers that cannot
+   *   sign events inline.
    */
-  async function loginWithNostr(pubkey: string, signature: string, event: any) {
+  async function loginWithNostr(
+    pubkey: string,
+    signature: string,
+    event: any,
+    preSignedAuthHeader?: string,
+  ) {
     isLoading.value = true
 
     // Mock Nostr login helper
@@ -271,12 +281,12 @@ export const useAuthStore = defineStore('auth', () => {
       }
 
       // Real API call — creates NIP-98 signed session via the active
-      // Nostr account in accountManager (set by useAccounts before this call)
-      const response = await authService.createNostrSession({
-        pubkey,
-        signature,
-        event,
-      })
+      // Nostr account in accountManager (set by useAccounts before this call).
+      // When preSignedAuthHeader is provided the signer is bypassed.
+      const response = await authService.createNostrSession(
+        { pubkey, signature, event },
+        preSignedAuthHeader,
+      )
 
       nostrPubkey.value = pubkey
       authType.value = 'nostr'