feat: add per-profile signer modes with persisted NIP-46 connections

Introduce three coexisting signing modes:
- Embedded (INTERNAL): vault-held nsec, decrypted in Rust, signs locally.
- Nip46Client (EXTERNAL): Keynctr is the NIP-46 CLIENT; the key never
  touches this machine.
- Nip46Bunker: legacy inverted mode (Keynctr as signer serving others).

Data model:
- StoredProfile gains signer_mode (serde-defaults to Embedded for legacy
  profiles); SignerMode moves from app.rs to vault.rs to break a circular
  dependency; app.rs re-exports it.
- Vault gains nip46_connections (profile-owned) and bumps VAULT_VERSION to
  3; migrate_vault_signer_modes() normalises on load (idempotent).
- Nip46Connection gains profile_npub ownership, parsed permissions,
  expires_at, and revoked_at.

Signer abstraction (src/signer):
- Signer trait gains pubkey_for() identity validation, a Signing enum
  (Local vs External) that re-verifies the returned event, and a permission
  surface (permissions/can_*/is_connection_valid) with safe defaults.
- permissions.rs: NIP-46 per-connection permission model (parse, validate,
  deny-by-default, no-broadening checks) with 52 unit tests.
- Nip46ClientSigner parses perms from nostrconnect:// URIs, enforces
  permissions on every gated request, persists/revokes connections in the
  vault, and audits permission denials via the app's audit log.
- App gains audit_log and a nip46_bunker_signer handle; default mode is
  Nip46Client (most secure).

Frontend: SignerModeScreen redesigned for the three modes with a
nostr-tools-based SignerManager client, new IPC allowlist entries, and
signer-mode styling.

Verified: cargo test --release 186 passed; clippy/fmt clean; frontend tsc
clean, vitest 110 passed.

frontend/package.json

@@ -27,6 +27,7 @@
     "dist": "npm run build && npm run electron:build && electron-builder --linux dir"
   },
   "dependencies": {
+    "nostr-tools": "^2.25.1",
     "react": "^18.3.1",
     "react-dom": "^18.3.1"
   },