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/capabilities/default.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Capability for the main window",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "opener:default"
+  ]
+}