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/home/src/main/kotlin/se/hagstedt/kvitt/feature/home/SyncStatusScreen.kt

@@ -134,6 +134,7 @@ private fun SyncStatusList(ui: SyncStatusUi.Ready, actions: SyncStatusActions, p
         if (summary.clockSkew) item(key = "clock") { ClockBanner(actions.onOpenDateSettings) }
         item(key = "status") { StatusCard(ui) }
         item(key = "sync-now") {
+            val syncing = ui.online && summary.running
             Column {
                 KvittButton(
                     stringResource(R.string.home_sync_status_sync_now),
@@ -141,7 +142,8 @@ private fun SyncStatusList(ui: SyncStatusUi.Ready, actions: SyncStatusActions, p
                     Modifier.fillMaxWidth(),
                     size = ButtonSize.Medium,
                     icon = KvittIcon.Sync,
-                    enabled = ui.canSyncNow,
+                    enabled = ui.canSyncNow || syncing,
+                    loading = syncing,
                 )
                 if (!ui.online) NeedsConnectionHint()
             }