refactor: migrate PDF export to desktop runtime abstraction

- Extract desktop-specific logic into src/services/desktop.ts
- Add runtime detection for Electron vs Tauri environments
- Simplify NoteEditor by delegating PDF export to desktop service
- Update printExport.ts to remove unused storage helpers
- Add Electron main process files in electron/ directory
- Update vite config and types for Electron integration
- Update .gitignore and package.json for Electron workflow

vite.config.ts

@@ -17,6 +17,10 @@ export default defineConfig(async () => ({
     port: 1420,
     strictPort: true,
     host: host || false,
+    headers: {
+      "Content-Security-Policy":
+        "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob: https: http:; font-src 'self' data:; connect-src 'self' https: http: ws: wss:; object-src 'none'; base-uri 'self'; frame-ancestors 'none'",
+    },
     hmr: host
       ? {
           protocol: "ws",