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.

src-tauri/tauri.conf.json

@@ -0,0 +1,33 @@
+{
+  "$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
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  }
+}