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.

build.rs

@@ -30,9 +30,15 @@ fn main() {
         assert!(status.success(), "blueprint-compiler failed");
     }
 
-    // Compile GSettings schema
+    // Compile GSettings schema into data/ so dev builds can find it
     let schema_file = data_dir.join("net.jeena.FeedTheMonkey.gschema.xml");
     println!("cargo:rerun-if-changed={}", schema_file.display());
+    let status = Command::new("glib-compile-schemas")
+        .arg(&data_dir)
+        .status()
+        .expect("failed to run glib-compile-schemas — is it installed?");
+    assert!(status.success(), "glib-compile-schemas failed");
+    println!("cargo:rustc-env=GSETTINGS_SCHEMA_DIR={}", data_dir.display());
 
     // Compile GResource
     let gresource_xml = data_dir.join("resources.gresource.xml");