feat: add LAN sharing hot reload controls

[GcsSloop]

Summary

• add LAN sharing settings, whitelist persistence, and backend access control
• hot-reload the desktop sidecar when LAN sharing toggles without changing desktop loopback access
• cover the change with backend, frontend, desktop, and local sidecar smoke tests

Verification

• cd backend && go test ./...
• cargo test --manifest-path desktop/src-tauri/Cargo.toml -q
• cargo test --manifest-path desktop/src-tauri/Cargo.toml -q lan_share_toggle_restarts_sidecar_without_changing_desktop_backend_addr -- --ignored
• npm --prefix frontend test -- --run src/features/settings/SettingsPage.test.tsx
• npm --prefix frontend run build