Add Tauri app skeleton and project files

Initialize the Nebula OS Tauri project: add frontend (src/) with HTML, JS, CSS and SVG assets, plus a sample greet UI. Add src-tauri Rust backend (Cargo.toml, build.rs, main/lib) with a greeting command, tauri.conf.json, capability schema and platform icons. Add package.json (dev dependency on @tauri-apps/cli), .gitignore files and VSCode recommended extensions. Expand README with project vision, architecture and phase roadmap to document the Windows-first development plan.

package.json

@@ -0,0 +1,12 @@
+{
+  "name": "nebula-os",
+  "private": true,
+  "version": "0.1.0",
+  "type": "module",
+  "scripts": {
+    "tauri": "tauri"
+  },
+  "devDependencies": {
+    "@tauri-apps/cli": "^2"
+  }
+}