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/electron/main.ts

@@ -38,12 +38,12 @@ protocol.registerSchemesAsPrivileged([
  * (HMR websocket included).
  */
 const CSP_PROD =
-  "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; " +
-  "connect-src 'self'; img-src 'self' data: https:; object-src 'none'; " +
+  "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; " +
+  "connect-src 'self'; img-src 'self' data: https:; font-src 'self' https://fonts.gstatic.com; object-src 'none'; " +
   "base-uri 'none'; form-action 'none'";
 const CSP_DEV =
-  "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; " +
-  "connect-src 'self' ws://localhost:* http://localhost:*; img-src 'self' data: https:; " +
+  "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; " +
+  "connect-src 'self' ws://localhost:* http://localhost:*; img-src 'self' data: https:; font-src 'self' https://fonts.gstatic.com; " +
   "object-src 'none'; base-uri 'none'; form-action 'none'";
 
 /** The CSP for a URL this window may load, or `null` for anywhere else. */
@@ -218,10 +218,26 @@ const RENDERER_METHODS: ReadonlySet<string> = new Set([
   'lock_vault',
   'remove_vault_password',
   'reveal_secret_key',
+  'export_secret_key',
+  // Legacy bunker
   'signer_connect',
   'signer_disconnect',
   'signer_status',
   'signer_approve',
+  // New signer modes (default: nip46_client most secure)
+  'signer_mode_get',
+  'signer_mode_set',
+  'embedded_signer_status',
+  'embedded_signer_approve',
+  'nip46_connect',
+  'nip46_disconnect',
+  'nip46_status',
+  'nip46_approve',
+  // Sidecar (local isolated signer, planned)
+  'sidecar_connect',
+  'sidecar_disconnect',
+  'sidecar_status',
+  'sidecar_approve',
 ]);
 
 /** True when `method` may be dispatched. Unknown methods never reach the backend. */