Security: store credentials in SecureStore

[colonelpanic8]

Changes

• Add expo-secure-store and configure plugin
• Store active credentials in SecureStore (with migration from legacy AsyncStorage keys)
• Store saved server passwords in SecureStore; saved server list no longer persists passwords in AsyncStorage
• Update background sync to read credentials via shared authStorage
• Adjust server edit UI to avoid prefilling password; leave blank to keep existing

Checks

• yarn typecheck
• yarn lint
• yarn prettier --check
• yarn test:unit