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.

.gitignore

@@ -0,0 +1,24 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
+dist
+dist-ssr
+*.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?