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

index.html

@@ -4,6 +4,10 @@
     <meta charset="UTF-8" />
     <link rel="icon" type="image/svg+xml" href="/vite.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta
+      http-equiv="Content-Security-Policy"
+      content="default-src 'self'; script-src 'self'; 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'"
+    />
     <title>Nextcloud Notes</title>
     <!-- Local fonts for offline support -->
     <link rel="stylesheet" href="/fonts/fonts.css">