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/api/rpc/mod.rs

@@ -109,7 +109,10 @@ const UNAUTHENTICATED_METHODS: &[&str] = &[
     "auth.login.totp",
     "auth.login.backup",
     "auth.isOnboardingComplete",
+    "auth.isSetup",
     "health",
+    // Onboarding restore (before user account exists)
+    "backup.restore-identity",
     // Inter-node RPC: called by federated peers over Tor, no session cookies
     "federation.peer-joined",
     "federation.peer-address-changed",
@@ -602,6 +605,7 @@ impl RpcHandler {
             "system.detect-usb-devices" => self.handle_system_detect_usb_devices().await,
             "system.disk-status" => self.handle_system_disk_status().await,
             "system.disk-cleanup" => self.handle_system_disk_cleanup().await,
+            "system.factory-reset" => self.handle_system_factory_reset(params).await,
 
             // Opt-in anonymous analytics
             "analytics.get-status" => self.handle_analytics_get_status().await,
@@ -646,6 +650,10 @@ impl RpcHandler {
                 let p = params.unwrap_or(serde_json::json!({}));
                 self.handle_backup_restore(&p).await
             }
+            "backup.restore-identity" => {
+                let p = params.unwrap_or(serde_json::json!({}));
+                self.handle_backup_restore_identity(&p).await
+            }
             "backup.delete" => {
                 let p = params.unwrap_or(serde_json::json!({}));
                 self.handle_backup_delete(&p).await