feat: factory reset, backup restore, auto-identity creation

- system.factory-reset RPC: wipes user data, preserves images/node_key
- Factory Reset button in Settings with confirmation modal
- backup.restore-identity RPC: decrypts and restores DID key
- Restore from Backup panel in OnboardingIntro first screen
- Auto-create default identity with Nostr key on boot if none exist

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/server.rs

@@ -48,6 +48,24 @@ impl Server {
         }
         state_manager.update_data(data.clone()).await;
 
+        // Auto-create default identity if none exist (fresh boot or factory reset)
+        {
+            let im = crate::identity_manager::IdentityManager::new(&config.data_dir).await;
+            if let Ok(mgr) = im {
+                if let Ok((list, _)) = mgr.list().await {
+                    if list.is_empty() {
+                        match mgr.create("Default".to_string(), crate::identity_manager::IdentityPurpose::Personal).await {
+                            Ok(record) => {
+                                let _ = mgr.create_nostr_key(&record.id).await;
+                                tracing::info!(did = %record.did, "Auto-created default identity with Nostr key");
+                            }
+                            Err(e) => tracing::debug!("Auto-identity creation (non-fatal): {}", e),
+                        }
+                    }
+                }
+            }
+        }
+
         // Revoke any previously published Nostr data (runs before publish so revocation is not overwritten)
         let identity_dir = config.data_dir.join("identity");
         let tor_proxy_revoke = config.nostr_tor_proxy.clone();