fix(cloud): enable keyring native backends so credentials persist

[proofofprints]

Problem

keyring = "3" with no backend feature → keyring 3.x silently uses an in-memory mock store, so cloud login does not survive an app restart (re-login every launch).

Fix

One-line Cargo.toml change enabling the platform-native backends:

keyring = { version = "3", features = ["windows-native", "apple-native", "sync-secret-service"] }

The save/load/restore code (cloud/auth.rs service popmanager-cloud, restore block in lib.rs) was already correct and wired.

Verification

• cargo check ✅ (Cargo.lock now pulls windows-native + secret-service backends)
• npx tsc --noEmit ✅
• Runtime: user to confirm creds land in Windows Credential Manager and survive restart (nothing to migrate — mock store was empty).

Part of the v1.6 alerts/UX polish batch.