scaffold: Epic 1 — project scaffold

Add the full Rust + GTK4 + libadwaita project skeleton:
- Cargo.toml with all dependencies (gtk4 0.11, libadwaita 0.9,
  webkit6 0.6, reqwest, serde, tokio, libsecret)
- build.rs that compiles Blueprint .blp files and bundles a GResource
- data/ui/window.blp — AdwApplicationWindow with AdwNavigationSplitView,
  sidebar with refresh button/spinner and primary menu,
  content page with article menu
- data/resources.gresource.xml bundling UI, HTML, and CSS
- data/net.jeena.FeedTheMonkey.gschema.xml with all GSettings keys
- html/content.html and html/content.css (minimal placeholders)
- src/main.rs, src/app.rs — AdwApplication with APP_ID net.jeena.FeedTheMonkey
- src/window.rs — AdwApplicationWindow GObject subclass loading the
  Blueprint template and persisting window size in GSettings
- COPYING (GPL-3.0) restored from master

The app compiles and the binary is ready to open a blank window.

src/main.rs

@@ -0,0 +1,7 @@
+mod app;
+mod window;
+
+fn main() -> glib::ExitCode {
+    let app = app::FeedTheMonkeyApp::new();
+    app.run()
+}