New Tauri commands: list_identities, create_identity, switch_identity,
delete_identity, import_identity_key, get_active_identity.
Settings UI: Identities section with list (active indicator, switch/delete
buttons), Create New Identity lightbox, Import Identity Key lightbox.
Export/Import buttons as placeholders for Phase 2/3.
Identity switch does hot-swap: tears down old Node, starts new one with
all background tasks, swaps AppNode RwLock. Frontend reloads after switch.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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