feat(auth): add resetStatusOnBackground config to keep session status on background

[PierreVieira]

Closes #1338

Problem

On Android, the lifecycle observer resets Auth.sessionStatus to SessionStatus.Initializing when the app moves to the background (onStop in Auth/src/androidMain/.../setupPlatform.kt), before it settles back to Authenticated on resume. Any consumer that renders UI from sessionStatus therefore observes a transient Initializing after every resume, which causes flicker — e.g. an authenticated user briefly appears as "loading"/"signed out".

This is Android-only: appleMain / jvmMain / webMain / linuxMain / mingwMain setupPlatform() just call initDone() and never reset to Initializing on background, so the status stays Authenticated across background/resume on those targets. The only existing opt-out, enableLifecycleCallbacks = false, is too coarse — it also disables pausing auto-refresh on background and reloading/refreshing the session on foreground.

Change

Add an opt-out config flag (additive, default = current behavior, no breaking change):

install(Auth) {
    resetStatusOnBackground = false
}

• true (default): unchanged — status is reset to Initializing on background.
• false: the last resolved status (e.g. Authenticated) is kept while auto-refresh is still paused on background; loadFromStorage() / refresh on resume continue to work exactly as before.

The onStop reset is now routed through an internal Auth.resetSessionStatusForBackground() helper (in Utils.kt, next to initDone()), so the behavior is unit-testable without driving ProcessLifecycleOwner.

Tests

Added two tests in AuthTest:

• default keeps current behavior (background → Initializing);
• with resetStatusOnBackground = false, the status stays Authenticated after backgrounding.

Notes

• Only affects Android, and only when enableLifecycleCallbacks = true.
• I didn't touch CHANGELOG.md since there's no unreleased section — happy to add an entry wherever you prefer.
• Alternative discussed in [Feature request]: Option to keep session status when app is backgrounded (don't reset to Initializing) #1338: simply not resetting to Initializing on background (making Android consistent with the other targets). I went with an opt-out flag to avoid any behavior change by default, but happy to switch approaches if you'd rather.