[Nostr] Add real multi-profile identity architecture

[shadowbipnode]

Goal

Replace the current single-profile model with true multi-profile Nostr identity management.

Current situation

Zap Browser now supports profile disconnect and re-import, but still uses a single active profile model internally.

Tasks

• Create multi-profile database schema.
• Support multiple encrypted Nostr private keys.
• Add active profile selection.
• Add profile switch UI.
• Ensure NIP-07 always uses the selected active profile.
• Clear or isolate permissions when switching profile.
• Migrate existing nostr_profile WHERE id=1 users.
• Prevent accidental cross-profile signing.

Acceptance criteria

• Users can store multiple Nostr identities.
• Users can switch active identity without restarting.
• Each profile remains encrypted independently.
• NIP-07 signs only with the active profile.
• Existing users migrate safely.