fix(session): keep feature-flag-derived state across logout

[bmc08gt]

Problem

Log out of an account with the Tipping beta flag on, log into another account, and you end up in a contradictory state: the beta flag is still on, but the scanner Tips tab is gone — and stays gone.

Root cause

SessionState.isTippingEnabled (and vibrateOnScan / showNetworkOffline) are fed only by featureFlagController.observe(flag) — hot StateFlows that dedup and re-emit only on a value change.

On logout (AuthState.LoggedOut in RealSessionController), stateHolder.reset() did a blanket _state.value = SessionState(), forcing isTippingEnabled = false. Because the observer's value was unchanged (the beta-flags store is device-level and shared across accounts, so tipping_enabled stays true), it never re-pushed to repopulate the field. isTippingEnabled was stuck at the reset default, so the scanner hid the Tips tab even though the flag was still enabled — a permanent desync until the flag value actually changed.

Fix

SessionStateHolder.reset() now clears only account-scoped state and preserves the device-scoped, feature-flag-derived fields (isTippingEnabled, vibrateOnScan, showNetworkOffline), letting observe(flag) remain their single source of truth:

• If the flag genuinely clears, the observer emits false and the field follows.
• If the flag persists, the field stays consistent with it.

Either way SessionState can no longer desync from the flag. Account/token/settings-derived fields (isPhoneNumberSendEnabled, hasGiveableBalance, autoStartCamera, …) still reset — their sources re-emit on account switch and self-heal.

Tests

SessionStateHolderTest: split the old reset test into reset clears account-scoped state, and added reset preserves device-scoped feature-flag state. All passing.

Note

Distinct root cause from #1157 (that fixes the persisted nav-bar order lacking Tips). This one fixes session state desyncing from the flag on logout. The two together cover both ways the Tips tab could fail to appear.