feat(app): add Tauri desktop build scaffold

Create src-tauri/ with tauri.conf.json (frameless transparent window,
1200x800 default, system tray), Cargo.toml with tauri v2 dependencies,
and main.rs with tray icon click-to-show and global shortcut
(Cmd+Shift+A) to toggle window visibility. Auto-updater plugin enabled.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src-tauri/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "aiui"
+version = "0.1.0"
+description = "AIUI Desktop Application"
+authors = ["AIUI"]
+edition = "2021"
+
+[build-dependencies]
+tauri-build = { version = "2", features = [] }
+
+[dependencies]
+tauri = { version = "2", features = [
+  "tray-icon",
+  "global-shortcut",
+] }
+tauri-plugin-updater = "2"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+[features]
+custom-protocol = ["tauri/custom-protocol"]

packages/app/src-tauri/build.rs

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

packages/app/src-tauri/src/main.rs

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+// Prevents additional console window on Windows in release.
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
+use tauri::{
+    tray::TrayIconBuilder,
+    Manager,
+};
+
+fn main() {
+    tauri::Builder::default()
+        .setup(|app| {
+            // Create system tray
+            let _tray = TrayIconBuilder::new()
+                .tooltip("AIUI")
+                .on_tray_icon_event(|tray, event| {
+                    use tauri::tray::TrayIconEvent;
+                    if let TrayIconEvent::Click { .. } = event {
+                        let app = tray.app_handle();
+                        if let Some(window) = app.get_webview_window("main") {
+                            let _ = window.show();
+                            let _ = window.set_focus();
+                        }
+                    }
+                })
+                .build(app)?;
+
+            // Register global shortcut (Cmd+Shift+A / Ctrl+Shift+A)
+            #[cfg(target_os = "macos")]
+            let shortcut = "CommandOrControl+Shift+A";
+            #[cfg(not(target_os = "macos"))]
+            let shortcut = "Ctrl+Shift+A";
+
+            let app_handle = app.handle().clone();
+            app.global_shortcut().on_shortcut(shortcut, move |_app, _shortcut, _event| {
+                if let Some(window) = app_handle.get_webview_window("main") {
+                    if window.is_visible().unwrap_or(false) {
+                        let _ = window.hide();
+                    } else {
+                        let _ = window.show();
+                        let _ = window.set_focus();
+                    }
+                }
+            })?;
+
+            Ok(())
+        })
+        .plugin(tauri_plugin_updater::Builder::new().build())
+        .run(tauri::generate_context!())
+        .expect("error while running tauri application");
+}

packages/app/src-tauri/tauri.conf.json

@@ -0,0 +1,56 @@
+{
+  "$schema": "https://raw.githubusercontent.com/nicolgit/tauri-docs/v2/tooling/cli/schema.json",
+  "productName": "AIUI",
+  "version": "0.1.0",
+  "identifier": "com.aiui.app",
+  "build": {
+    "frontendDist": "../dist",
+    "devUrl": "http://localhost:5173",
+    "beforeDevCommand": "pnpm dev:app",
+    "beforeBuildCommand": "pnpm build:app"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "AIUI",
+        "width": 1200,
+        "height": 800,
+        "minWidth": 800,
+        "minHeight": 600,
+        "decorations": false,
+        "transparent": true,
+        "resizable": true,
+        "center": true
+      }
+    ],
+    "security": {
+      "csp": "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; connect-src 'self' https: wss:"
+    },
+    "trayIcon": {
+      "iconPath": "icons/icon.png",
+      "iconAsTemplate": true
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ],
+    "macOS": {
+      "minimumSystemVersion": "10.15"
+    }
+  },
+  "plugins": {
+    "updater": {
+      "active": true,
+      "endpoints": [],
+      "pubkey": ""
+    },
+    "global-shortcut": {}
+  }
+}