Temporary setup of alternative to advertisements

lib/services/ad_config.dart

@@ -0,0 +1,17 @@
+/// The single switch that turns real AdMob ads on.
+///
+/// While this is `false`, the app behaves in every observable way as though
+/// ads were live — each placement is still *decided* on exactly the same
+/// schedule (new-user grace periods, earned credits, the ad-free purchase,
+/// the sync gate blocking uploads) — but where a real ad would appear, it
+/// shows `AdPlaceholderScreen` instead, and the Google Mobile Ads SDK is
+/// never initialized at all. No ad requests go out, so none of them can
+/// come back "Account not approved yet" or no-fill.
+///
+/// That means the pacing and gating can be exercised and tuned for real
+/// before AdMob approval lands.
+///
+/// Flipping this to `true` is the only change needed to go live: every
+/// placement picks up its real ad unit from [AdService] and the placeholder
+/// stops being reachable. Nothing else in the app branches on it.
+const bool adsEnabled = false;