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/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "nebula-os",
+  "private": true,
+  "version": "0.1.0",
+  "type": "module",
+  "scripts": {
+    "dev": "tauri dev",
+    "build": "tauri build",
+    "tauri": "tauri"
+  },
+  "devDependencies": {
+    "@tauri-apps/cli": "^2"
+  },
+  "dependencies": {
+    "@nebulaproject/core": "^0.1.3",
+    "@tauri-apps/api": "^2.10.1"
+  }
+}