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-tauri/tauri.conf.json

@@ -0,0 +1,39 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "productName": "nebula-os",
+  "version": "0.1.0",
+  "identifier": "com.user.nebula-os",
+  "build": {
+    "frontendDist": "../src"
+  },
+  "app": {
+    "withGlobalTauri": true,
+    "windows": [
+      {
+        "title": "nebula-os",
+        "width": 800,
+        "height": 600,
+        "fullscreen": true,
+        "decorations": false
+      }
+    ],
+    "security": {
+      "csp": null,
+      "assetProtocol": {
+        "enable": true,
+        "scope": ["$APPDATA/**"]
+      }
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  }
+}