M7g: feature cleanups after the design-system fix pass

Resource-backed snackbars (showKvittSnackbar) in account, onboarding and
settings; SettingsScaffold replaced by KvittScaffold + SettingsMessageEffect.
KvittButton(loading) for S-6 saving and S-16 Synka nu. S-10 missing state
disables Synka nu offline (connectivity in the ViewModel, new
detail_missing_offline goldens). S-6 system back asks to discard unsaved
changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

android/feature/account/build.gradle.kts

@@ -10,6 +10,8 @@ roborazzi {
 }
 
 dependencies {
+    // BackHandler: system back on S-6 with unsaved changes asks to discard (design-spec S-6).
+    implementation(libs.androidx.activity.compose)
     testImplementation(libs.kotlinx.coroutines.test)
     testImplementation(libs.androidx.compose.ui.test.junit4)
     testImplementation(libs.robolectric)