scaffold: compile GSettings schema at build time for dev runs

build.rs now runs glib-compile-schemas on data/ so that debug builds
can find the schema without a system-wide install. main.rs sets
GSETTINGS_SCHEMA_DIR from the build-time constant when running in
debug mode.

src/main.rs

@@ -2,6 +2,12 @@ mod app;
 mod window;
 
 fn main() -> glib::ExitCode {
+    // In development builds, point GSettings at the locally compiled schema.
+    // In release/installed builds the schema is found via the system path.
+    if cfg!(debug_assertions) {
+        std::env::set_var("GSETTINGS_SCHEMA_DIR", env!("GSETTINGS_SCHEMA_DIR"));
+    }
+
     let app = app::FeedTheMonkeyApp::new();
     app.run()
 }