The setContext() method was setting prefs but not storing context as a class member variable, causing generateDeviceIdentity() to always see null context.

- OpenClawApp now calls setContext() to enable file-based identity
- NodeService uses lazy delegate to get NodeClient from OpenClawApp
- This ensures both Activity and Service use the same device identity
- generateDeviceIdentity() now skips if context is null (avoids crash)

Root cause: SharedPreferences are not shared between processes (Activity vs Service),
causing new keypair generation on every restart, resulting in 'device signature invalid'.

Fix: Use file-based storage (device_identity.json) instead of SharedPreferences.
File is shared across all processes in the same app.