Add Endurain sync backend

[DanyPM]

Push body weight + composition to a self-hosted Endurain server via POST /api/v1/health/weight (upsert-by-date).

Auth: OAuth2 username/password login -> JWT access + rotating refresh token, stored in EncryptedSharedPreferences (no password kept), with proactive refresh in connect() and reactive refresh-on-401. MFA supported; SSO/PKCE deferred. Implemented clean-room from Endurain's REST API (not derived from Gadgetbridge's AGPL client).

Field mapping reads openScale's generic, unit-aware values: masses (muscle_mass/bone_mass) sent in kg, fat/water in %, plus visceral_fat. bmi is left for the server to auto-calc; source is omitted (server enum rejects non-garmin). Export only.

[oliexdev]

Thanks for the Endurain backend — the mapping logic and the token-refresh handling look solid, and the test coverage on buildWeightRequest is appreciated. 👍

One thing I'd like changed before merging: the androidx.security:security-crypto dependency for token storage.

Why:

1. It's deprecated. Jetpack Security Crypto (EncryptedSharedPreferences / MasterKey) was deprecated as of 1.1.0-alpha07 and never reached a stable release — the 1.1.0-alpha06 pin here is the same never-stabilized library, one version before the deprecation annotation landed. It's also known for keyset corruption crashes on some OEM devices, and there's no official AndroidX drop-in successor (Google's guidance is to roll your own Keystore + Cipher).

2. It's inconsistent with the rest of the app. Every other backend stores its secrets as plaintext putString in the shared openScaleSyncSettings prefs — MQTT password (MQTTViewModel), Wger token (WgerViewModel), InfluxDB token + password (InfluxDbViewModel), Webhook auth header (WebhookViewModel). There's no encryption anywhere else in the codebase. Since minSdk is 31, app-private storage is already sandboxed and file-based-encrypted at rest, so encrypting only the Endurain tokens adds a dead dependency and a one-off pattern for marginal real-world benefit.

So could you please drop the EncryptedSharedPreferences and store the tokens like everything else?

[DanyPM]

Hi, thank you for the cosntructive feedback.

I have updated the PR per your suggestions. Feel free to squash commits.