Add mode READMEs and move Bigscreen app

Add comprehensive READMEs for Bigscreen and Desktop modes, and relocate the Tauri frontend prototype into a new Bigscreen/ subdirectory. This moves package.json/package-lock, src, src-tauri, assets, views, styles and related files into Bigscreen/ to separate the controller-first shell from top-level docs. Update the root README.md to reframe the project as "NebulaOS", describe Bigscreen/Desktop modes, the vision, tech stack and development notes. This reorganizes the repo layout for clearer mode separation and documentation.

Bigscreen/src/core/router.js

@@ -0,0 +1,34 @@
+export const createRouter = (outlet) => {
+  const views = new Map();
+  let current = null;
+
+  const register = (view) => {
+    views.set(view.id, view);
+  };
+
+  const navigate = (id) => {
+    const view = views.get(id);
+    if (!view) {
+      throw new Error(`Unknown view: ${id}`);
+    }
+
+    current = id;
+    outlet.innerHTML = view.render();
+    const nextView = outlet.querySelector(".view");
+    if (nextView) {
+      requestAnimationFrame(() => {
+        nextView.classList.add("view-entered");
+      });
+    }
+    view.mount?.(outlet);
+    return view.getNavigationContract();
+  };
+
+  const getCurrent = () => current;
+
+  return {
+    register,
+    navigate,
+    getCurrent,
+  };
+};