Keep the dark theme and shown tips across sign-in and sign-out

[dadachi]

Phase 2 follow-up (bug fixes). Port of NativeAppTemplate-Android #109, bug 1. (Bug 2 of that PR, the skipped push cleanup on account deletion, does not apply: the Free edition has no push notifications.)

Sign-in, sign-out and account deletion reset the device settings

clearUserPreferences() did toBuilder().clear(). It runs on every sign-in (SignInEmailAndPasswordViewModel clears before storing the new session), every sign-out (LoginRepositoryImpl.logout, on success and on failure) and on account deletion (ShopkeeperEditViewModel.deleteShopkeeper). So each of these reset the dark-theme choice to "System Default" and showed the "Tap shop below" tip again.

Fix: renamed to clearSession(). It resets the proto to its defaults except the two device settings. Decision per field of Free's user_preferences.proto:

Field(s)	Kept?	Why
dark_theme_config	kept	A device/display setting chosen on this phone. Not account data.
did_show_tap_shop_below_tip	kept	A one-time "how to use the UI" tip. It holds no shopkeeper data, and the iOS app shows the same tip with TipKit, which is per device and survives sign-out.
id, account_id, personal_account_id, account_owner_id, account_name, email, name, time_zone, token, client, uid, expiry, is_logged_in	cleared	The session and the shopkeeper's identity.
permissions, android_app_version, should_update_app/privacy/terms, shop_limit_count	cleared	Come from GET /accounts/:id/permissions for the signed-in account and are fetched again after sign-in. Keeping them would show the previous shopkeeper's limit or update prompts until that fetch finishes.
locale	cleared	The shopkeeper's server-side language (AppLanguageSyncer compares against it). The app language itself is per-app/system, not stored here.
did_show_you_are_in_personal_account_alert	cleared	A per-session flag (unused in Free, kept in the proto).
is_email_updated, is_my_account_deleted, is_shop_deleted	cleared	One-shot snackbar triggers about the previous shopkeeper's actions. The two set around sign-out (is_email_updated, is_my_account_deleted) are set after the clear, as before, so their snackbars still work (checked on the device, below).

Free has no device_id, organization or invitation fields, so those rows of the paid table do not apply.

Tests (each seen red before the fix)

• NativeAppTemplatePreferencesDataSourceTest.clearSession_keepsTheDeviceSettings: red with the old toBuilder().clear(). Replaces clearUserPreferences_resetsEverythingToTheDefaults, which pinned the bug as "current behavior".
• NativeAppTemplatePreferencesDataSourceTest.clearSession_forgetsEverythingAboutTheShopkeeper: checks every per-account field against the proto3 defaults (stored locale ja before the clear, so the locale check is meaningful). Red when I made the clear also keep is_shop_deleted.
• LoginRepositoryImplTest.logout_keepsTheDarkTheme: red with the old clear.
• NativeAppTemplatePreferencesDataSourceWriteFailureTest: the log message is now "Failed to clear the session" (red until the message changed).
• The rest is the rename (clearSessionCallCount in TestLoginRepository, etc.).

Local checks

• ./scripts/pre-push (spotless, lint, R8 shrink, unit tests): passed. 400 unit tests, 0 failures.
• ./gradlew :app:assembleRelease: passed.

Device check (Pixel, com.nativeapptemplate.nativeapptemplatefree debug build, local API)

System set to light mode so the app's Dark is visible.

• Signed in as shopkeeper1, set Dark, signed out: the welcome and sign-in screens rendered dark.
• Signed up and signed in as a throwaway shopkeeper: Shops rendered dark and Settings showed Dark. Dismissed the "Tap shop below" tip.
• Changed the email in Edit Profile: the app signed out and the "confirmation link has been sent to your new email address" snackbar showed on the welcome screen.
• Signed in again: still Dark, and the tip stayed dismissed.
• Deleted the account: the "Account deleted successfully." snackbar showed on the welcome screen, and the server record is gone.
• Signed in as shopkeeper1: Settings still showed Dark, no tip. Then set the theme back to System Default and the system back to dark mode.

🤖 Generated with Claude Code

https://claude.ai/code/session_01K5hZEFqXhBsUQwSWsgR6eH