feat: implement Installer Studio MSIX authoring workflow

Add the shared Rust engine, Tauri UI and CLI, registry authoring, developer signing, branded start screen and .isproj persistence. Remove the unused project ID.

Validation: frontend build and tests, Rust workspace tests, native build and real Windows SDK package generation passed.

Closes #2
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Closes #7

src/apps/desktop/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "msix-studio-desktop",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "vite --host 127.0.0.1 --port 1420 --strictPort",
+    "build": "tsc --noEmit && vite build",
+    "test": "node --experimental-strip-types --test tests/*.test.mjs",
+    "tauri": "tauri"
+  },
+  "dependencies": { "@tauri-apps/api": "^2.0.0" },
+  "devDependencies": { "@tauri-apps/cli": "^2.0.0", "typescript": "^5.9.0", "vite": "^7.0.0" }
+}