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/backup/mod.rs

@@ -6,4 +6,4 @@
 mod identity;
 pub mod full;
 
-pub use identity::create_encrypted_backup;
+pub use identity::{create_encrypted_backup, restore_encrypted_backup};