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/.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Generated by Tauri
+# will have schema files for capabilities auto-completion
+/gen/schemas

src-tauri/Cargo.toml

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+[package]
+name = "nebula-os"
+version = "0.1.0"
+description = "A Tauri App"
+authors = ["you"]
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[lib]
+# The `_lib` suffix may seem redundant but it is necessary
+# to make the lib name unique and wouldn't conflict with the bin name.
+# This seems to be only an issue on Windows, see https://github.com/rust-lang/cargo/issues/8519
+name = "nebula_os_lib"
+crate-type = ["staticlib", "cdylib", "rlib"]
+
+[build-dependencies]
+tauri-build = { version = "2", features = [] }
+
+[dependencies]
+tauri = { version = "2", features = [] }
+tauri-plugin-opener = "2"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+

src-tauri/build.rs

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+fn main() {
+    tauri_build::build()
+}

src-tauri/capabilities/default.json

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

src-tauri/src/lib.rs

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+// Learn more about Tauri commands at https://tauri.app/develop/calling-rust/
+#[tauri::command]
+fn greet(name: &str) -> String {
+    format!("Hello, {}! You've been greeted from Rust!", name)
+}
+
+#[cfg_attr(mobile, tauri::mobile_entry_point)]
+pub fn run() {
+    tauri::Builder::default()
+        .plugin(tauri_plugin_opener::init())
+        .invoke_handler(tauri::generate_handler![greet])
+        .run(tauri::generate_context!())
+        .expect("error while running tauri application");
+}

src-tauri/src/main.rs

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+// Prevents additional console window on Windows in release, DO NOT REMOVE!!
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
+fn main() {
+    nebula_os_lib::run()
+}

src-tauri/tauri.conf.json

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+{
+  "$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"
+    ]
+  }
+}